Great informative thread!
I'm very much in the situation as a few here. Have 19 years work from UK and living here since 1992 so Irish pension all good. I got my forecast from UK a while back for £101 a week or £111 if I contribute up until 66. I've also been accepted to pay class 2 but it's not too easy to get this put in place plus I wanted to find out what difference it would make to my payments if I did pay the £2670 back payments. I've been trying to phone them for days now to find out this.... must have tried 20 times and it just hangs up after all the recorded messages and now today there's industrial action doh! I found another number yesterday and spoke to someone from the NI contributions department and they said I should definitely speak to someone in the future pensions department to find out if it will make a difference before I start paying anything.
But on the bright side I didn't even know I could claim a UK pension on top of my Irish one until 4 or 5 years ago!