There used to be a few Saturday morning dealers in Donegal Pass (in Belfast) but it's been years since I was there. One guy (sadly passed away) used to buy in bins of coins from England and I could sit happily for a few hours on a Saturday going through them looking for interesting stuff. My second ever best buy was a 1870 Japenese 5 sen in EF+ that I got out of a 10p bin. (Probably £300+ today)
Check out the local house clearance auctions up round the NW. There must be some up there. Four times out of 5 there will be nothing but the odd time there will be some interesting coins that will be worth looking at. Build up a relationship with the auction house so you can just give them a ring to see if any coins have come in to save you needless journeys.
Aviod "antique fairs" like the plague - there is nothing there, but there are some good coin fairs around. There should be one soon in Dublin run by Whytes (with an auction of some very nice, but very pricy stuff). It is definitely worth a journey - I'm tempted to go again myself this year..