Following a string of defeats during Black Week in early December 1899, the British government realised that it would need more troops than just the Regular Army to fight the Second Boer War, particularly mounted units.

After a slow railway journey, the yeomanry companies assembled at Umtali in late June, by which time sickness was rife and there had been a number of deaths.

While the leading brigades of the RFF pushed south into Transvaal, the two Yeomanry battalions did not reach Bulawayo until the beginning of September.

[11] The war now developed into a drawn-out phase of Guerrilla warfare, with the British mounted troops engaged in 'drives' across the veldt, forcing the Boers against lines of blockhouses.

As late as February 1902, 17th Battalion formed part of a column in a coordinated series of drives across North East Orange Free State.

[14][15] The Marquess of Waterford, a former lieutenant in the Royal Horse Guards, was appointed as commanding officer on 10 February 1902 with the rank of lieutenant-colonel.

However, the Treaty of Vereeniging had been signed while they were at sea, and Lord Waterford soon came home, resigning his captain's commission on 25 August 1902,[21] and returning to command the new South Irish regiment.

[14][25][26] Prior to World War I the South Irish Horse was attached to the Regular 3rd Cavalry Brigade at The Curragh.

[35][38][39][40] When the German Spring Offensive was launched on 21 March 1918, 16th (Irish) Division was holding a slight salient, with its Forward Zone stretched across a series of spurs.

The Official History records that, 'two companies of 7th Battalion Royal Irish Regiment, posted in forward zones, suffered terribly; not a man succeeded in escaping.

The dark green peaked cap had a red band and in service dress was won with a khaki cover; a full-dress head-dress was considered but never adopted.

[14][15][51] The Honorary Colonel of the regiment, appointed on 19 March 1904, was Field Marshal Prince Arthur, Duke of Connaught and Strathearn.