I have MANY favourites...
Kinnegar in Donegal have some fantastic beers: Scraggy Bay, Crossroads, Limeburner and Rustbucket to name but four.
White Hag in Sligo don't do a bad beer. Their Atlantean and the Little Fawn session ale are fab.
Wicklow Wolf are outstanding. Elevation Pale Ale, Mammoth Pale Ale and for me the finest red ale in the country Wildfire Hoppy.
O'Hara's in Carlow do the best stout, their Nitro Stout which is smooth, creamy and tasty.
Trouble Brewing changed their lineup of beers in recent years. They used to do a gorgeous Sabotage Golden Ale which seems to have been discontinued. Their Ambush is a nice beer though.