I found the following DOB for Sy(lvester) 1836, Cat(herine) 1838, Ann 1840 & 1849, James 1844, Mary 1843 & 1852, all in St. Mary's, Drogheda. The double DOB for some of the names suggests there were at least two McGrath families in Drogheda during this era - likely a sibling to Peter.
Sources indicate that marriage rcds for this RC parish don't begin until 1870. This MAY indicate they simply haven't been transcribed. Suggest you contact parish officials.