The South Irish Horse existed for a mere 20 years, formed in 1902 as the South of Ireland Imperial Yeomanry and disbanded in July 1922 along with five other famous Irish Regiments. Raised as The South of Ireland Imperial Yeomanry in 1902, it can trace its roots to various companies of Imperial Yeomanry. The 61st (South Irish) Company, 17th Battalion, Imperial Yeomanry being the most obvious. This company was raised on the 7th March 1900.
The declaration of war against Germany in August 1914 found the South Irish Horse at summer camp, as was its sister regiment the North Irish Horse. Together they supplied a composite regiment who acted as GHQ (General Head Quarters) troops in the BEF (British Expeditionary Force) in France, B Squadron from the South Irish Horse and two from the North Irish Horse.
Briefly, the regiment served as separate divisional cavalry squadrons and then as 2 corps cavalry regiments. In September 1917 the officers and men of both regiments were retrained as infantry and formed 7th (South Irish Horse) Battalion Royal Irish Regiment. The battalion was posted to 49th Infantry Brigade in 16th (Irish) Division. A look at the list of war dead will show 21 men recorded as ‘Killed in Action’ on 12th December 1917 and a further 9 ‘Died of Wounds’ by the end of the month. All of these men had previously been in the South Irish Horse.
In Terance Denman's book "Ireland's Unknown Soldiers", he states that actually 22 men were killed and over 40 wounded by one artillery shell at St Emilie on 13th December 1917 and not on the 12th. It is understandable for the official record to be occasionally incorrect, especially when considering it records over 700 000 dead from the British Isles alone.
On 21st March 1918 the battalion was caught in the maelstrom of the German Kaiserschlacht (Kaiser's Battle) offensive. The Official History records that, "2 Coys of 7/Royal Irish posted in forward zone suffered terribly; not a man succeeded in escaping." 77 officers and men of 7th (SIH) Royal Irish Regiment were killed in action that day, 42 were formerly South Irish Horse. 14 officers of the battalion were captured that day, 6 of whom were formerly South Irish Horse. By the end of the month over 90 men of the battalion were dead or dying. According to the War Diary of 49th Infantry Brigade the battalion strength on 30th March 1918 was 1 officer and 34 other ranks. The battalion strength on 20th March is not known but a sister battalion (2nd Royal Irish Regiment) in the brigade had a strength of 18 officers and 514 men before the battle and 1 officer and 31 other ranks on 30th March. 15 officers of this battalion were also captured (two of whom were also formerly South Irish Horse). Far more details can be found in "Orange Green and Khaki" by Tom Johnstone.
In April 1918 the battalion was reduced to a cadre and reformed in June with 835 men from other regiments. It retained South Irish Horse in its title but must have had very few former members in its numbers. In the battalion from April until the end of the war one man was killed in action and two died of wounds who were previously South Irish Horse and 16 former South Irish Horse died.