Bunsen is the gold standard - it's absolutely perfect, it blows the competition away so thoroughly that in my opinion it's closer to the best burger in Europe than the best burger in Ireland.
Failing that, Wowburger is probably second best. Five Guys is also very tasty in fairness - a bit expensive, but tasty.
I'm not a huge fan of the likes of GBK and that - too many options, and the burgers they serve tend to emphasize pure size and a million toppings over quality ingredients and actual taste. There's no point in a burger you can't even fit into your mouth - or one dripping in BBQ sauce or with a black pudding crowbarred in to make it more Oirish for the tourists.