A cavalry regiment of the Special Reserve
August 1914 : in Belfast. Part of 3rd Cavalry Brigade.
On mobilisation in August 1914, the regiment was split up.
A Squadron moved to France on 19 August 1914, and was attached to GHQ,BEF. On 4 January 1916, it was transferred to 55th Division, and on 10 May 1916 to VII Corps, forming the 1st North Irish Horse together with D and E Squadrons. This unit transferred to XIX Corps in July 1917, and to V Corps in September 1917. In March 1918, the unit became a Cyclist Battalion, which it then remained.
B Squadron was attached to the 59th (2/North Midland) Division in August 1915, leaving in June 1916 to form, with C Squadron and the Service Squadron of the 6th (Inniskilling) Dragoons, the 2nd North Irish Horse. This unit was attached to X Corps until August 1917, when broken up. The personnel went to infantry training, and eventually more than 300 of them joined the 9th (Serrvice) Bn, the Royal Irish Fusiliers.
C Squadron moved to France on 22 August 1914, and was attached to GHQ,BEF. On 14 April 1915 it was transferred to 3rd Division, and in June 1916 joined B in the 2nd North Irish Horse.
D Squadron was attached to 51st (Highland) Division in early 1915, and in June 1916 joined A in the 1st North Irish Horse.
E Squadron was attached to 34th Division from early 1915, and in June 1916 joined A in the 1st North Irish Horse.
F Squadron was attached to the 33rd Division from early 1915 until April 1916. It was briefly attached to 1st Cavalry Division, 49th (West Riding) Division, and 32nd Division, before joining X Corps in June 1916. It was redesignated B Squadron in May 1916.