At the outbreak of war, the Regiment was based in Dublin and was immediately mobilised but very soon split up. The Squadrons served mainly attached to Divisions until 1916, before being attached to a Corps. After September 1917, the original Regiment served as infantry and three new Squadrons were formed.
A Squadron joined 21st Division during 1915. In May 1916, it left and went to form part of XV Corps Cavalry Regiment and, during January 1917, it became XVIII Corps Cavalry Regiment.
B Squadron moved to France as GHQ Troops in August 1914 and joined 2nd Division in May 1915, before leaving the division during May 1916 to form part of I Corps Cavalry Regiment. At that time, it was redesignated as S Squadron
C Squadron joined 16th (Irish) Division during 1915 and, a year later, left the division to form part of I Corps Cavalry Regiment.
E Squadron moved to France and joined 39th Division during March 1916, before leaving the division in May to form part of I Corps Cavalry Regiment.
F Squadron moved to France in May 1917 and replaced a squadron of the Hertfordshire Yeomanry within XVIII Corps Cavalry Regiment.
S Squadron joined 32nd Division in 1915 and, during May 1916, it left the division to form part of XV Corps Cavalry Regiment. At that time, It was redesignated as B Squadron and, in November 1916, it moved to IX Corps Cavalry Regiment. It then became XVIII Corps Cavalry Regiment in early 1917.
In 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, at that time, merged with 2nd South Irish Horse.
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. It amalgamated with 1st South Irish Horse at the end of August 1917 and was then converted into infantry and became 7th Battalion (South Irish Horse), Royal Irish Regiment in September 1917.
1914 - 2.
1915 - 5.
1916 - 10.
1917 - 20.
1918, up to 11th Nov - 2.