They will usually search a 5 year window for a single fee. The population of NI is less than 2 million. The numbers of events they register is a fraction of the rest of the UK. So searching isn't so hard.
I have been in GRONI and heard a member of the public ringing in, looking for an event around X date. The staff found it in a few minutes, and the customer paid over the phone by credit card. I think you have to buy a certified copy if you do that. I think that's £15, but GRONI will tell you the correct charges.