Off the top of my head there is the Frank Harte memorial week-end in Dublin (mostly trad singing) and the Cork Folk Festival in September. Probably many others. Given the way the economy here has gone recently nothing is certain to go ahead this year, but I think those two are fairly safe bets. I'll check out some dates and come back with more details. If your start date is a little bit flexible then the All Ireland Fleadh Cheoil held in Tullamore this year on the UK August Bank Holiday week-end could be worth checking out.