The three Ps on the 51a were P250, 262 and 266. They lasted until 1971, I thought they were w/d in late 71, perhaps they were there a bit longer. I think P250 may have gone before the others. The 51a was always double deck after the Ps left. Unfortunately, I don't have w/d dates for them.
There were 5 provincial Ps in Clontarf in 1966 due to a shortage of buses, but these were w/d when new double deckers came in 1967. They were used as extras on all routes, and would also have served on the 51a, although 250, 262 and 266 would have been the regulars even then.