Yes, Frank's family. I'm related through Joseph's family and as far as I know they were all in the Chorley area until around the 1960's. Pleased to meet a distant relation!
The 1951 census has John aged 0 born in Ireland.
The Killashandra records have both Prior and McPartlan families in the 1941 register.
There's a Mary McPartlan aged 12 born 1828, which fits well. Parents: Michael & Eleanor McPartlan.
The listing I'm looking at for Patrick and Anne Prior has a Patt Prior aged 14 born 1828, its on Family Search, but I can't see the actual record - is it possible there's a page missing? The other part of the record has two Prior deaths.
Maybe there's another copy of this census? I will try and have a look.
I looked in the parish registers for a marriage, or baptism of John, there were other Priors, but its possible I could have missed it, I find the handwriting hard to decipher. I had a quick scout in neighbouring parishes in case there was overlap, but no joy.
I'd be glad to hear what you discover!
All the best, regen