Calico, I still think you are a FTB, as you have never bought. When you say that you can regain your FTB status it sounds a bit misleading since I think the only circumstances that allow a person to be a FTB again are in the case of a divorce.
The way I see it is, if you buy somewhere else as your PPR, before the 6 year period specified, then you would have to pay the inheritence tax. Either that, or you could purchase a 2nd home, but it could not be your PPR (i.e. you could not live in it) so it would have to be classified as an investment property, thereby incurring stamp-duty. In any case, when you purchase you would lose your FTB status.
Regarding weighing up the buying vs renting, if it is only a temporay move to Dublin, then I suggest to rent. The interest you would pay on the mortgage for a property in Dublin over a 3 year period will almost certainly exceed the cost of rent during that same period. The fact that you would lose your FTB status for a temporary move is also a major negative towards the case for buying.
Also, be careful here because from my understanding of it if you move out of the property you inherited, then you would be liable for all of the inheritance tax. Get professional advice if in doubt.