Dunraven was one of the committee who raised the first Sharpshooters in 1899/1900 for service in the Boer War. He declined a senior rank, serving instead as a captain and then being seriously ill for much of the campaign.
Following the Regiment’s success on active service, Dunraven was given permission to raise a regiment for service at home and on 23 July 1901, the 3rd County of London (Sharpshooters) Imperial Yeomanry was formally embodied.
Dunraven was the first commanding officer, becoming Honorary Colonel in 1904 and remaining in the post until 1925. The regimental colours of Royal Irish green, yellow and mauve are reputedly based on his racing colours.