The parish of Clashmore is within the civil registration covered by Youghal. There is just 1 Patrick M’Grath birth registered there in 1868, so that may be the one you for your Patrick Joseph McGrath. The birth was registered in Youghal 1868, Volume 4, page 1062.If you order a copy of that it should give confirm his parents and the townland (address) they lived at which may enable you to trace back further.
Birth, death and RC marriage registration only started in 1864 in Ireland so for the earlier births you would need baptism records. Likewise for the parents marriage. According to this site, the records for that parish go back to 1811.
Griffiths Valuation for Waterford, 1851, has just tow Patrick McGrath households listed in Clashmore parish. One in the townland of Shanacoole and the other in Ticknock. The birth cert and the parish records may establish whether either is your family home.