The Ulster Defence Regiment: Background
The UDR was an infantry regiment of the British Army established in 1970, raised through public appeal, newspaper and television advertisements, with the official role of defending life and property in Northern Ireland against armed attack or sabotage. At the time of formation it was the largest infantry regiment in the British Army. It consisted mostly of part-time volunteers until 1976, when a full-time cadre was added.
In 1992, the UDR merged with the Royal Irish Rangers to form the Royal Irish Regiment, becoming the new unit’s Home Service battalions. In 2006, the regiment was retrospectively awarded the Conspicuous Gallantry Cross in recognition of its services.

Why the numbers? The county system
Each battalion was tied to a specific county or city of Northern Ireland, following a long Irish military tradition. On formation the UDR consisted of: 1st (County Antrim) Battalion HQ Ballymena; 2nd (County Armagh) Battalion HQ Armagh; 3rd (County Down) Battalion HQ Ballykinlar; 4th (County Fermanagh) Battalion HQ Enniskillen; 5th (County Londonderry) Battalion HQ Londonderry; 6th (County Tyrone) Battalion HQ Omagh; 7th (City of Belfast) Battalion HQ Holywood. The battalions’ titles were abbreviated to their number followed by “UDR”.
Four more were added within two years, taking the total to eleven: 8th (County Tyrone), 9th (County Antrim), 10th (City of Belfast) and 11th (Craigavon).

The 7/10 UDR and 1/9 UDR  date from after the 1984 battalion mergers, when manpower reductions forced several battalions to combine.