I would anticipate, from both my own experience and those of others in this forum, the preferable option for HMRC is some sort of documented continuity of proof that your were contributing to NI in Ireland immediately after you left the UK. But if you did indeed cease self employment on leaving the UK and commenced employment immediately on return to Ireland then that is the preferable scenario. I suspect any indication of a break in that through unemployment, education or such would push you into the higher Class 3 contribution category rather than the Class 2. If you are happy enough to accept this higher cost for the 5 odd years you want to buy back, I'd say fire away by indicating a break in the continuity. . However you might be able to avail of the lower class 2 contributions for the years you can prove you were in full time employment in Ireland. It'll be down really to whoever is assessing your application in HMRC