August 1914 : based in Dublin, the regiment was strictly a unit of the Special Reserve. No Territorial Force regiments of the Yeomanry were established in Ireland and the South Irish Horse provided a mounted form of part-time soldiering. It was mobilised on declaration of war but very soon split up. The squadrons served mainly attached to Divisions until 1916, and then attached to a Corps. After September 1917, the original regiment served as infantry and three new squadrons were formed.
A Squadron joined 21st Division in 1915. On 11 May 1916 it left and went to form part of XV Corps Cavalry Regiment. On 16 January 1917 it became XVIII Corps Cavalry Regiment.
B Squadron moved to France as GHQ Troops on 17 August 1914, one of the very few non-regular units to form part of the original BEF. Joined 2nd Division on 4 May 1915. On 15 May 1916 it left and went to form part of I Corps Cavalry Regiment (and was redesignated S Squadron).
C Squadron joined 16th (Irish) Division in 1915. On 17 May 1916 it left and went to form part of I Corps Cavalry Regiment.
E Squadron moved to France and joined 39th Division on 17 March 1916. On 17 May 1916 it left and went to form part of I Corps Cavalry Regiment.
F Squadron moved to France on 18 May 1917 and replaced a squadron of the Hertfordshire Yeomanry in XVIII Corps Cavalry Regiment.
S Squadron joined 32nd Division in 1915. On 14 May 1916 it left and went to form part of XV Corps Cavalry Regiment. It was then redesignated B Squadron. On 21 November 1916 it moved to IX Corps Cavalry Regiment, and in January 1917 it became XVIII Corps Cavalry Regiment.
On 17 June 1916, C, E and S Squadrons, now together again in I Corps Cavalry Regiment, became known as 1st South Irish Horse. It left I Corps in August 1917, and merged with 2nd South Irish Horse (see below).
2nd South Irish Horse was formed from A and B Squadrons, together with B Squadron of the Hertfordshire Yeomanry, and had been XVIII Corps Cavalry Regiment. Amalgamated with 1st South Irish Horse at the end of August 1917.
Converted into infantry and became the 7th (Service) Battalion (South Irish Horse), the Royal Irish Regiment on 1 September 1917.