Thanks to you both for the replies.
I have looked back at my bills over the past while and it makes sense for me to move, just wasn't sure if the process was as simple as it is supposed to be. Any financial savings would be quickly negated if the service is not up to scratch.
I switched to Meteor and its had an unexpected effect on my mobile costs every month. Since their coverage is not really great in many parts of the country you save loads on call charges as you can neither make nor receive phone calls! Great service..
Although to give them their due they are far cheaper than the other two as long as you never have to go outside of a major urban area. The new roaming agreement with O2 might help.
Switching my number across was no problem and I expect no problems when I switch back to O2 (even though I expect to end up paying a lot more).
Haven't had a problem w. Meteor phone in the last two years; with the exception of rural Mayo/Galway there isn't anywhere I've been where I couldn't use my phone. O2 agreement should sort that one out fairly soon. They are still doing the free sms if you top up by €20 if you sign up by (I think) 30th Sept.
In case anyone else is thinking of switching, I did get round to making the move a few days ago and all went according to plan. Switched over to the new network with the same number in about an hour. Looks like the system works as well in practice as it does in theory.