The regiment that the 6th Battalion, Ulster Defence Regiment grouped into had who as their oldest predecessor?
6th Battalion, Ulster Defence Regiment:  It was, along with the rest of the regiment, amalgamated with the Royal Irish Rangers in 1992 to form the Royal Irish Regiment. Royal Irish Regiment (1992):  Their oldest predecessor; the 27th Regiment of Foot; was first raised in June 1689 to fight in the Williamite War in Ireland.
the 27th Regiment of Foot