i went about 7 years ago, was not asked for any Id as far as i can remember. i dont think that you need a passport to travel from mainland Britain to any part of Ireland at the moment but may change soon
just found this which may be of help http://news.bbc.co.uk/1/hi/northern_ireland/70 59580.stm
You don't need a passport. You need ID with your picture on to fly with easyjet. I go to Belfast a few times a year, in fact I was there for Xmas and on one trip the only photo ID I had was a railcard, which they accepted.