I think it depends on which airline you are flying with as Aer Lingus only require photoid for adults and will accept a bus pass as id. British Midland will accept work id and neither of these airlines require photoid for kids. I havent flown Ryanair recently as they are crap and I hate them and AL and BM are cheaper from London anyway.
I did have fun last Christams tho when got to Ireland and the guy at the passport desk asked for our passports and I only had mine and dps. He wanted to see dd's and I said that she didnt have one and didnt need one. He said that the rules had changed but hed let me through this once (too bloody right). Not sure what planet he was operating on cos the rule hasnt changed at all.