Hi, Ken;
I don't see a Patrick McGrath in the Roots Ireland records with the birth or baptism date of 18 Mar 1812, which is not unusual. My Margaret "Nelly" Brien McGrath would have been about 19 years old, if Patrick was her son. I have the impression that Thomas Francis McGrath was her firstborn, since there are no other births listed in the Roots Ireland records before his birth.
Irish records are challenging, as we all know!!
Noreen Cullen