I am using Irish Central Bank Rates to calculate value in Euro for foreign currencies. For example, if you buy something in UK and you have invoice/receipt in Sterling (£), you can find the date of purchase in table below and use the rate to calculate Euro value for accounts purposes.

For example, if you pay by credit card the better way is to use the rate on your Credit card statement. If you pay by cash and you exchanged Eur to Sterling, you can use rate the Exchange office charged you. Otherwise, if you cannot identify the exchange rate, you can use table below.

FX rate = foreign exchange rate

Source:
Foreign exchange rates from Central Bank of Ireland from 2008 to present (MS Excel document)

Foreign exchange rates 2016

January February March April May
June July August September October
November December
Pound sterling US dollar Czech koruna Polish zloty
Date GBP USD CZK PLN
4 Jan 16 0.73810 1.0898 27.023 4.2955
5 Jan 16 0.73235 1.0746 27.022 4.3084
6 Jan 16 0.73440 1.0742 27.026 4.3374
7 Jan 16 0.74545 1.0868 27.030 4.3512
8 Jan 16 0.74519 1.0861 27.022 4.3523
11 Jan 16 0.74705 1.0888 27.021 4.3646
12 Jan 16 0.75080 1.0836 27.021 4.3568
13 Jan 16 0.75020 1.0816 27.021 4.3458
14 Jan 16 0.75703 1.0893 27.021 4.3730
15 Jan 16 0.76150 1.0914 27.021 4.4129
18 Jan 16 0.76263 1.0892 27.037 4.4606
19 Jan 16 0.76473 1.0868 27.023 4.4384
20 Jan 16 0.77000 1.0907 27.042 4.4702
21 Jan 16 0.77182 1.0893 27.064 4.4943
22 Jan 16 0.75459 1.0808 27.026 4.4587
25 Jan 16 0.75890 1.0815 27.021 4.4690
26 Jan 16 0.76095 1.0837 27.021 4.4942
27 Jan 16 0.75965 1.0888 27.025 4.4710
28 Jan 16 0.76228 1.0903 27.021 4.4534
29 Jan 16 0.76410 1.0920 27.026 4.4398
Date GBP USD CZK PLN
1 Feb 16 0.76100 1.0884 27.021 4.3952
2 Feb 16 0.75860 1.0919 27.026 4.4055
3 Feb 16 0.75330 1.0933 27.025 4.3982
4 Feb 16 0.76595 1.1206 27.021 4.4152
5 Feb 16 0.76975 1.1202 27.037 4.4134
8 Feb 16 0.77240 1.1101 27.062 4.4352
9 Feb 16 0.77945 1.1236 27.058 4.4530
10 Feb 16 0.77328 1.1257 27.031 4.4289
11 Feb 16 0.78740 1.1347 27.069 4.4485
12 Feb 16 0.77735 1.1275 27.063 4.4109
15 Feb 16 0.77200 1.1180 27.037 4.3945
16 Feb 16 0.77580 1.1166 27.029 4.4062
17 Feb 16 0.77835 1.1136 27.044 4.3979
18 Feb 16 0.77143 1.1084 27.030 4.3817
19 Feb 16 0.77715 1.1096 27.023 4.3777
22 Feb 16 0.78243 1.1026 27.022 4.3646
23 Feb 16 0.77985 1.1002 27.028 4.3668
24 Feb 16 0.78935 1.0981 27.047 4.3680
25 Feb 16 0.78920 1.1027 27.052 4.3581
26 Feb 16 0.78745 1.1006 27.064 4.3631
29 Feb 16 0.78580 1.0888 27.057 4.3543
Date GBP USD CZK PLN
1 Mar 16 0.77800 1.0872 27.056 4.3468
2 Mar 16 0.77410 1.0856 27.046 4.3297
3 Mar 16 0.77435 1.0901 27.057 4.3339
4 Mar 16 0.77483 1.0970 27.058 4.3313
7 Mar 16 0.77438 1.0953 27.056 4.3279
8 Mar 16 0.77608 1.1028 27.058 4.3275
9 Mar 16 0.77135 1.0973 27.047 4.3149
10 Mar 16 0.76728 1.0857 27.037 4.2921
11 Mar 16 0.77595 1.1090 27.059 4.3099
14 Mar 16 0.77428 1.1119 27.050 4.2837
15 Mar 16 0.78358 1.1109 27.043 4.2889
16 Mar 16 0.78730 1.1064 27.050 4.3023
17 Mar 16 0.78218 1.1311 27.044 4.2895
18 Mar 16 0.77855 1.1279 27.035 4.2625
21 Mar 16 0.78303 1.1271 27.035 4.2581
22 Mar 16 0.78790 1.1212 27.037 4.2612
23 Mar 16 0.78985 1.1171 27.038 4.2563
24 Mar 16 0.78938 1.1154 27.064 4.2788
29 Mar 16 0.78450 1.1194 27.073 4.2498
30 Mar 16 0.78580 1.1324 27.075 4.2538
31 Mar 16 0.79155 1.1385 27.051 4.2576
Date GBP USD CZK PLN
1 Apr 16 0.79890 1.1432 27.030 4.2440
4 Apr 16 0.79738 1.1380 27.046 4.2340
5 Apr 16 0.80160 1.1367 27.036 4.2448
6 Apr 16 0.80808 1.1336 27.029 4.2636
7 Apr 16 0.80728 1.1364 27.025 4.2833
8 Apr 16 0.80730 1.1363 27.022 4.2953
11 Apr 16 0.80060 1.1390 27.026 4.2793
12 Apr 16 0.79840 1.1396 27.030 4.2923
13 Apr 16 0.79388 1.1298 27.036 4.2869
14 Apr 16 0.79560 1.1252 27.028 4.2992
15 Apr 16 0.79575 1.1284 27.025 4.2967
18 Apr 16 0.79780 1.1306 27.022 4.3059
19 Apr 16 0.78975 1.1343 27.021 4.3027
20 Apr 16 0.79100 1.1379 27.021 4.2875
21 Apr 16 0.78693 1.1355 27.024 4.3085
22 Apr 16 0.78335 1.1263 27.046 4.3506
25 Apr 16 0.77640 1.1264 27.047 4.3911
26 Apr 16 0.77483 1.1287 27.027 4.3799
27 Apr 16 0.77475 1.1303 27.034 4.3918
28 Apr 16 0.77838 1.1358 27.045 4.3880
29 Apr 16 0.78025 1.1403 27.038 4.3965
Date GBP USD CZK PLN
2 May 16 0.78248 1.1493 27.048 4.3833
3 May 16 0.79103 1.1569 27.037 4.3844
4 May 16 0.79284 1.1505 27.031 4.3936
5 May 16 0.78860 1.1439 27.028 4.4234
6 May 16 0.78850 1.1427 27.021 4.4198
9 May 16 0.78998 1.1395 27.022 4.4142
10 May 16 0.78760 1.1375 27.023 4.4201
11 May 16 0.78958 1.1409 27.022 4.4244
12 May 16 0.78888 1.1389 27.023 4.4193
13 May 16 0.78758 1.1348 27.021 4.3929
16 May 16 0.78800 1.1324 27.023 4.3718
17 May 16 0.78150 1.1318 27.022 4.3621
18 May 16 0.77485 1.1279 27.022 4.3885
19 May 16 0.76585 1.1197 27.023 4.3928
20 May 16 0.77008 1.1219 27.025 4.4203
23 May 16 0.77443 1.1215 27.033 4.4367
24 May 16 0.76513 1.1168 27.025 4.4358
25 May 16 0.75925 1.1146 27.028 4.4205
26 May 16 0.75970 1.1168 27.023 4.4108
27 May 16 0.76245 1.1168 27.028 4.3912
30 May 16 0.76125 1.1139 27.021 4.3940
31 May 16 0.76185 1.1154 27.022 4.3865
Date GBP USD CZK PLN
1 Jun 16 0.77358 1.1174 27.027 4.3978
2 Jun 16 0.77335 1.1188 27.029 4.3903
3 Jun 16 0.77285 1.1154 27.025 4.3861
6 Jun 16 0.78720 1.1349 27.023 4.3776
7 Jun 16 0.77880 1.1348 27.021 4.3486
8 Jun 16 0.78048 1.1378 27.021 4.3228
9 Jun 16 0.78438 1.1343 27.021 4.3321
10 Jun 16 0.78480 1.1304 27.026 4.3591
13 Jun 16 0.79515 1.1268 27.035 4.3884
14 Jun 16 0.79478 1.1225 27.048 4.4332
15 Jun 16 0.79158 1.1230 27.073 4.4119
16 Jun 16 0.79033 1.1174 27.067 4.4490
17 Jun 16 0.78770 1.1254 27.069 4.4463
20 Jun 16 0.77375 1.1332 27.063 4.3925
21 Jun 16 0.76755 1.1314 27.064 4.3991
22 Jun 16 0.76793 1.1283 27.071 4.3785
23 Jun 16 0.76595 1.1389 27.062 4.3571
24 Jun 16 0.80750 1.1066 27.103 4.4550
27 Jun 16 0.83400 1.0998 27.151 4.4619
28 Jun 16 0.82720 1.1073 27.108 4.4413
29 Jun 16 0.82550 1.1090 27.114 4.4261
30 Jun 16 0.82650 1.1102 27.131 4.4362
Date GBP USD CZK PLN
1 Jul 16 0.83830 1.1135 27.093 4.3966
4 Jul 16 0.83905 1.1138 27.095 4.4304
5 Jul 16 0.84997 1.1146 27.071 4.4457
6 Jul 16 0.85300 1.1069 27.078 4.4562
7 Jul 16 0.85085 1.1080 27.049 4.4275
8 Jul 16 0.85243 1.1070 27.033 4.4278
11 Jul 16 0.85097 1.1049 27.037 4.4210
12 Jul 16 0.84275 1.1092 27.034 4.4171
13 Jul 16 0.83398 1.1072 27.033 4.4055
14 Jul 16 0.83311 1.1157 27.049 4.4115
15 Jul 16 0.83275 1.1128 27.040 4.4099
18 Jul 16 0.83372 1.1053 27.024 4.3805
19 Jul 16 0.83950 1.1035 27.014 4.3729
20 Jul 16 0.83605 1.1013 27.022 4.3758
21 Jul 16 0.83595 1.1015 27.025 4.3692
22 Jul 16 0.84108 1.1014 27.023 4.3554
25 Jul 16 0.83670 1.0982 27.025 4.3578
26 Jul 16 0.83710 1.0997 27.021 4.3629
27 Jul 16 0.83875 1.0991 27.035 4.3733
28 Jul 16 0.84220 1.1090 27.045 4.3636
29 Jul 16 0.84400 1.1113 27.031 4.3630
Date GBP USD CZK PLN
1 Aug 16 0.84540 1.1164 27.029 4.3510
2 Aug 16 0.84310 1.1193 27.032 4.3241
3 Aug 16 0.83910 1.1200 27.034 4.3130
4 Aug 16 0.84603 1.1136 27.027 4.2940
5 Aug 16 0.84810 1.1156 27.022 4.2841
8 Aug 16 0.84955 1.1087 27.027 4.2786
9 Aug 16 0.85399 1.1078 27.027 4.2669
10 Aug 16 0.85541 1.1184 27.025 4.2684
11 Aug 16 0.86038 1.1153 27.020 4.2604
12 Aug 16 0.86050 1.1158 27.018 4.2658
15 Aug 16 0.86677 1.1180 27.023 4.2695
16 Aug 16 0.87060 1.1295 27.020 4.2696
17 Aug 16 0.86640 1.1276 27.023 4.2907
18 Aug 16 0.86073 1.1321 27.024 4.2925
19 Aug 16 0.86537 1.1326 27.022 4.2992
22 Aug 16 0.86240 1.1306 27.024 4.3019
23 Aug 16 0.85995 1.1339 27.023 4.3126
24 Aug 16 0.85130 1.1268 27.022 4.3039
25 Aug 16 0.85570 1.1290 27.026 4.3123
26 Aug 16 0.85450 1.1290 27.024 4.3244
29 Aug 16 0.85476 1.1170 27.023 4.3314
30 Aug 16 0.85170 1.1168 27.025 4.3436
31 Aug 16 0.84806 1.1132 27.026 4.3533
Date GBP USD CZK PLN
1 Sep 16 0.84050 1.1146 27.025 4.3645
2 Sep 16 0.84260 1.1193 27.022 4.3914
5 Sep 16 0.83643 1.1156 27.021 4.3541
6 Sep 16 0.83535 1.1159 27.021 4.3388
7 Sep 16 0.83968 1.1237 27.021 4.3186
8 Sep 16 0.84560 1.1296 27.021 4.3103
9 Sep 16 0.84558 1.1268 27.022 4.3345
12 Sep 16 0.84475 1.1226 27.024 4.3494
13 Sep 16 0.84763 1.1247 27.021 4.3478
14 Sep 16 0.85078 1.1218 27.023 4.3420
15 Sep 16 0.85198 1.1254 27.023 4.3254
16 Sep 16 0.85203 1.1226 27.024 4.3167
19 Sep 16 0.85500 1.1165 27.021 4.2925
20 Sep 16 0.86213 1.1184 27.022 4.2975
21 Sep 16 0.85693 1.1150 27.022 4.2915
22 Sep 16 0.85925 1.1238 27.021 4.2852
23 Sep 16 0.86435 1.1214 27.022 4.2869
26 Sep 16 0.87090 1.1262 27.021 4.3045
27 Sep 16 0.86413 1.1220 27.025 4.2887
28 Sep 16 0.86208 1.1225 27.023 4.2893
29 Sep 16 0.86138 1.1221 27.022 4.3065
30 Sep 16 0.86103 1.1161 27.021 4.3192
Date GBP USD CZK PLN
3 Oct 16 0.87318 1.1236 27.021 4.2933
4 Oct 16 0.87540 1.1161 27.021 4.2936
5 Oct 16 0.88155 1.1211 27.028 4.2941
6 Oct 16 0.88323 1.1185 27.021 4.2928
7 Oct 16 0.90353 1.1140 27.021 4.2813
10 Oct 16 0.90070 1.1160 27.021 4.2821
11 Oct 16 0.90233 1.1079 27.021 4.2712
12 Oct 16 0.89990 1.1020 27.025 4.2974
13 Oct 16 0.90430 1.1038 27.021 4.3020
14 Oct 16 0.89910 1.1002 27.029 4.3020
17 Oct 16 0.90485 1.0994 27.024 4.3281
18 Oct 16 0.89565 1.0993 27.021 4.3184
19 Oct 16 0.89338 1.0979 27.021 4.3180
20 Oct 16 0.89568 1.0980 27.021 4.3282
21 Oct 16 0.89320 1.0886 27.021 4.3251
24 Oct 16 0.89015 1.0891 27.021 4.3166
25 Oct 16 0.88998 1.0872 27.021 4.3042
26 Oct 16 0.89465 1.0925 27.021 4.3245
27 Oct 16 0.89158 1.0927 27.022 4.3292
28 Oct 16 0.89905 1.0922 27.022 4.3307
31 Oct 16 0.90050 1.0946 27.024 4.3278
Date GBP USD CZK PLN
1 Nov 16 0.90173 1.1025 27.024 4.3085
2 Nov 16 0.90063 1.1095 27.022 4.3255
3 Nov 16 0.88658 1.1064 27.021 4.3186
4 Nov 16 0.88808 1.1093 27.021 4.3188
7 Nov 16 0.89040 1.1062 27.021 4.3301
8 Nov 16 0.89088 1.1038 27.021 4.3283
9 Nov 16 0.89030 1.1022 27.027 4.3388
10 Nov 16 0.87785 1.0895 27.022 4.3588
11 Nov 16 0.86133 1.0904 27.054 4.4081
14 Nov 16 0.85985 1.0777 27.026 4.4060
15 Nov 16 0.86638 1.0765 27.030 4.4245
16 Nov 16 0.86110 1.0702 27.036 4.4433
17 Nov 16 0.86055 1.0717 27.050 4.4448
18 Nov 16 0.86218 1.0629 27.035 4.4429
21 Nov 16 0.85725 1.0631 27.032 4.4307
22 Nov 16 0.85423 1.0617 27.044 4.4205
23 Nov 16 0.85393 1.0602 27.027 4.4154
24 Nov 16 0.84800 1.0548 27.034 4.4239
25 Nov 16 0.85182 1.0592 27.039 4.4190
28 Nov 16 0.85303 1.0588 27.044 4.4194
29 Nov 16 0.84815 1.0576 27.045 4.4299
30 Nov 16 0.85250 1.0635 27.060 4.4483
Date GBP USD CZK PLN
1 Dec 16 0.84098 1.0627 27.061 4.4676
2 Dec 16 0.84303 1.0642 27.058 4.4892
5 Dec 16 0.84168 1.0702 27.057 4.5005
6 Dec 16 0.84210 1.0734 27.045 4.4780
7 Dec 16 0.85095 1.0730 27.036 4.4280
8 Dec 16 0.84995 1.0762 27.038 4.4540
9 Dec 16 0.83935 1.0559 27.027 4.4454
12 Dec 16 0.83900 1.0596 27.027 4.4530
13 Dec 16 0.83488 1.0610 27.022 4.4442
14 Dec 16 0.83963 1.0644 27.023 4.4402
15 Dec 16 0.83598 1.0419 27.021 4.4450
16 Dec 16 0.83890 1.0439 27.021 4.4200
19 Dec 16 0.84140 1.0422 27.021 4.4110
20 Dec 16 0.83978 1.0364 27.021 4.4095
21 Dec 16 0.84240 1.0421 27.021 4.4082
22 Dec 16 0.84645 1.0444 27.021 4.4096
23 Dec 16 0.85278 1.0446 27.023 4.4098
27 Dec 16 0.85300 1.0445 27.021 4.4090
28 Dec 16 0.85128 1.0401 27.042 4.4028
29 Dec 16 0.85300 1.0453 27.021 4.4141
30 Dec 16 0.85618 1.0541 27.021 4.4103

Source:
Foreign exchange rates from Central Bank of Ireland from 2008 to present (MS Excel document)