Yes it seems in these smaller communities that there were limited people to marry and you often see a brothers/sisters marrying brothers and sisters of another family and if the Sheehan family had a few members its likely they married cousins (having done a Swiss branch which has extensive records I have the same gg x ? grandparents several times over , eg their ggg grandchildren married each other, so I can see that happening here as well...however I don't know what Anne Sheehan's parents were and other family members and can't link it in!!
The Irish side is so frustrating as records are pretty much dead from 1800 back (destroyed) and I have several other Irish lines where I am certain people are related but further back but as there are no records I can't confirm.

Looking at the valuations (I think in 1855) there was James, Francis and Michael Shaughnessy living in the area so it is POSSIBLE they came from one of them BUT having TWO Patrick Shaughnessy's born the same time makes it more solid.
As I said I do know another Shaughnessy (Francis) went to the USA but Michael, Thomas and James all stayed in the area (1901 and 1911 census), although some children of Michael and Catherine Mulready went to the USA as well (but appear to have gone over between 1901 and 1911 and I think ended up around New York according to emigration records)). Francis ended up around Indiana I think (well his children did).

Where in the USA did your Patrick end up?
Claire