Good morning, Campbellking (and Robbie):
Given that the NI Executive's website provides a link for obtaining
home testing lateral flow kits, it appears that (unlike people travelling to England from, say, Italy) it's not necessary to get tested at a pharmacy (in order to to obtain a certificate to prove that you've tested negative). Simply carrying out a test at home should suffice:
https://www.nidirect.gov.uk/articles/coronavirus-covid-19-travelling-within-common-travel-area
(If you're reporting your lateral flow tests online, you'll have both a text message and an email on your phone to acknowledge your result in the unlikely chance of anyone asking for confirmation that you've submitted a negative result).