David Lenehan has written a great round-up of the Top Irish Web Apps over on Read/Write Web. He seems to have hit on most of the usual suspects (LouderVoice, PutPlace, Nooked, Roam4Free, Pixenate, etc.).
However, the fact that David missed out on a number of up and coming web apps like dbTwang and Touristr only goes to show how difficult it is to keep up with the pace of innovation in Ireland right now.
In the tech world, Ireland is a hot happening place right now.