I assume you're not referring to modern neo-pagans, and you mean at what point did Christianisation become total in Ireland and Britain?

I suppose the unhelpful answer was that paganism never disappeared from Celtic culture. Even now, Celtic Christianity is full of pagan symbolism (the Brigid's Cross, one of the main symbols of Christianity in Ireland, is a pagan symbol) and the Gaelic name of Ireland, Éire, is derived from the pagan god Éirú.

The answer for when most of Ireland (I can't speak for Britain) became basically Christianised is not known for sure, but it's thought paganism in Ireland had been more or less absorbed by Christianity by the 8th century, and you'd assume earlier in Britain due to its geographical location. I'll try to find a source for you now, but long before that Ireland had become a source of missionary Christianity in the rest of Europe.

One of the major events that signalled the passing of Celtic religion into Christianity was the Synod of Whitby (which actually took place in Northumbria in Britain), in which the Celtic calendar was subsumed into the Roman Catholic calendar, essentially aligning the church in Ireland and Celtic Britain with the church in Rome.

That's a significant event because it shows that the church in Ireland was so strong that it was brought into the control and orthodoxy of the central church.

As for your question of when the last true pagans would have lived, there's no way to know as far as I can see because it's not documented, but you can extrapolate from the historical events that it was probably no later than the 8th century.