From the outbreak of the War to early 1916, six North Irish Horse Squadrons saw service beyond Irish shores.

A Squadron landed at Le Havre on 19 August 1914, serving as GHQ Troops until 4 January 1916, then as Divisional Cavalry to the 55th Division until 10 May 1916.

C Squadron landed in France on 22 August 1914. Initially serving as Divisional Cavalry to the 5th Division (16 Brigade), the Squadron joined A Squadron at GHQ on 12 October 1914. From 14 April 1915 to 10 May 1916 it served as Divisional Cavalry to the 3rd Division, then briefly with the 49th (West Riding) Division and 36th (Ulster) Division.

B Squadron moved from Derry to Antrim in December 1914 and in August 1915 was attached to the 59th Division in Hertfordshire. In May 1916 it was re-designated as F Squadron and apparently returned to Antrim.

D Squadron joined the 51st (Highland) Division at Bedford before sailing for France, arriving at Le Havre on 2 May 1915. The Squadron served with the 51st Division until May 1916.

F Squadron moved to Salisbury Plain in the first half of 1915 to join the 33rd Division as Divisional Cavalry. The Squadron landed at Le Havre on 17 November 1915, serving with the 33rd Division until 16 May 1916, when it joined the 49th (West Riding) Division and then the 32nd Division four days later. On 25 May 1916 it was re-designated as B Squadron.

E Squadron joined the 34th Division as Divisional Cavalry in June 1915, landing at Le Havre on 12 January 1916.

During the first year or so or the War, individual cavalry Squadrons had been assigned to Army Divisions. In mid-1916 this was replaced with a system of Corps Cavalry Regiments, each Regiment comprising three squadrons and a headquarters.

On 10 May 1916, A, D and E Squadrons of the North Irish Horse formed the 1st Regiment North Irish Horse, which was attached to the Army's VII Corps. On 21 June 1916 the 2nd Regiment North Irish Horse was formed from C and B (formerly F) Squadrons of the North Irish Horse, and the 6th Iniskilling Dragoons (Service Squadron), and was attached to the Army's X Corps. (The Inniskilling Dragoon Squadron was formed in October 1914 from volunteers of the Inniskilling Horse of the Ulster Volunteer Force, and had arrived in France on 6 October 1915, serving as Divisional Cavalry to the 36th (Ulster) Division).

In 1917, in order to supplement depleted infantry units, it was agreed that six of the twenty cavalry regiments attached to infantry corps be disbanded and the men be trained as infantrymen.

In August 1917 the 2nd Regiment North Irish Horse was dismounted, most troopers going to base for infantry training while a party accompanied their horses to Egypt to hand over to the Australians before returning for infantry training. In September and October the troops were absorbed into the 9th (Service) Battalion Royal Irish Fusiliers (108th Brigade, 36th (Ulster) Division), which henceforth became the 9th (North Irish Horse) Battalion Royal Irish Fusiliers. The 9th Battalion had seen much severe action since its arrival in France in late 1915. Most recently it has suffered heavy losses during 3rd Ypres (Passchendaele). On 16 August at Langemarck the Battalion lost 36 killed, 323 wounded, 12 cases of shell-shock, 83 missing, and 2 missing believed killed.

From February to March 1918 the V Corps Cyclist Regiment (North Irish Horse) was formed from the dismounted 1st Regiment North Irish Horse. Cyclist Regiments were some 25 per cent smaller than Cavalry Regiments, and therefore a number of officers and men were transferred to other units. At the same time, the former V Corps Cyclist Battalion was broken up, with one officer and 24 other ranks transferred-in.