Question: The regiment that the 6th Battalion, Ulster Defence Regiment grouped into had who as their oldest predecessor?

Context paragraphs:
7th Battalion, Ulster Defence Regiment: 7th (City of Belfast) Battalion, Ulster Defence Regiment (7 UDR) was formed in 1970 as part of the 7 original battalions specified in The Ulster Defence Regiment Act 1969, which received Royal Assent on 18 December 1969 and was brought into force on 1 January 1970.  It was amalgamated with the 10th Battalion, Ulster Defence Regiment in 1984 to form the 7th/10th (City of Belfast) Battalion, Ulster Defence Regiment.

2nd/11th Battalion, Ulster Defence Regiment: The 2nd//11th (County Armagh) Battalion, Ulster Defence Regiment was formed in 1991 as a result of an amalgamation between the 2nd Battalion Ulster Defence Regiment and the 11th Battalion Ulster Defence Regiment.  The resultant 2/11 UDR was subsumed into the Royal Irish Rangers in 1992 as part of the Options for Change amalgamations and was renamed the 6th Battalion Royal Irish Regiment.

4th/6th Battalion, Ulster Defence Regiment: The 4th/6th (County Fermanagh and County Tyrone) Battalion, Ulster Defence Regiment was formed in 1991 as a result of an amalgamation between the 4th Battalion, Ulster Defence Regiment and the 6th Battalion, Ulster Defence Regiment.  The resultant 4/6 UDR was subsumed into the Royal Irish Rangers in 1992 as part of the amalgamation which formed the Royal Irish Regiment.

4th Battalion, Ulster Defence Regiment: 4th (County Fermanagh) Battalion, Ulster Defence Regiment (4 UDR) was formed in 1970 as part of the 7 original battalions specified in The Ulster Defence Regiment Act 1969, which received Royal Assent on 18 December 1969 and was brought into force on 1 January 1970.  It was amalgamated with the 6th Battalion, Ulster Defence Regiment in 1992 to form the 4th/6th Battalion, Ulster Defence Regiment.

6th Battalion, Ulster Defence Regiment: The 6th (County Tyrone) Battalion, Ulster Defence Regiment (6 UDR) was formed in 1970 as part of the seven original battalions specified in The Ulster Defence Regiment Act 1969, which received Royal Assent on 18 December 1969 and was brought into force on 1 January 1970.  It was, along with the rest of the regiment, amalgamated with the Royal Irish Rangers in 1992 to form the Royal Irish Regiment.

1st Battalion, Ulster Defence Regiment: 1st (County Antrim) Battalion, Ulster Defence Regiment (1 UDR) was formed in 1970 as part of the 7 original battalions specified in The Ulster Defence Regiment Act 1969, which received Royal Assent on 18 December 1969 and was brought into force on 1 January 1970.  It was amalgamated with the 9th Battalion, Ulster Defence Regiment in 1984 to form the 1st/9th Battalion, Ulster Defence Regiment.

2nd Battalion, Ulster Defence Regiment: 2nd (County Armagh) Battalion, Ulster Defence Regiment (2 UDR) was formed in 1970 as part of the seven original battalions specified in the Ulster Defence Regiment Act 1969, which received Royal Assent on 18 December 1969 and was brought into force on 1 January 1970.  It was, along with the rest of the regiment, amalgamated with the Royal Irish Rangers in 1992 to form the Royal Irish Regiment.  It had previously been amalgamated in 1991 with the 11th Battalion Ulster Defence Regiment to form the 2nd/11th Battalion Ulster Defence Regiment.

Royal Irish Regiment (1992): The Royal Irish Regiment (27th (Inniskilling) 83rd and 87th and Ulster Defence Regiment) (R IRISH) is an infantry regiment of the British Army.  The regiment was founded in 1992 through the amalgamation of the Royal Irish Rangers and the Ulster Defence Regiment.  Their oldest predecessor; the 27th Regiment of Foot; was first raised in June 1689 to fight in the Williamite War in Ireland.  Other notable regiments in their lineage include the Royal Inniskilling Fusiliers, Royal Irish Rifles and the Royal Irish Fusiliers (Princess Victoria's).

7th/10th Battalion, Ulster Defence Regiment: The 7th/10th (City of Belfast) Battalion, Ulster Defence Regiment was formed in 1984 as a result of an amalgamation between the 7th Battalion Ulster Defence Regiment and the 10th Battalion Ulster Defence Regiment.  The resultant 7/10 UDR was subsumed into the Royal Irish Rangers in 1992 as part of the amalgamation which formed the Royal Irish Regiment.

11th Battalion, Ulster Defence Regiment: The 11th (Craigavon) Battalion, Ulster Defence Regiment was formed from companies of the 2nd Battalion Ulster Defence Regiment (2 UDR) and the 3rd Battalion Ulster Defence Regiment in 1972.  In 1991 under the reductions planned in Options for Change by the British Army, it again amalgamated with 2 UDR to form the 2nd/11th Battalion Ulster Defence Regiment.

Supporting paragraphs: 6th Battalion, Ulster Defence Regiment Royal Irish Regiment (1992)
Answer: the 27th Regiment of Foot