In 1885, he stood again for Parliament and was elected as a Conservative for the North Armagh constituency.[3] By this stage, he had become a prominent figure in the Orange Order and in the unionist political movement. He retained his North Armagh seat in the 1886 election. He was subsequently involved in organising the establishment of the Irish Unionist Alliance (IUA), a political party which sought to unite the unionist movement across Ireland. He became the IUA's first leader in 1891, a position which he held until his death. Saunderson became known for his uncompromising speeches in the House of Commons, and he was nicknamed "the Dancing Dervish" by friends and opponents. He was invested as a member of Her Majesty's Most Honourable Privy Council in 1898 in recognition of his political service.[4]
Saunderson had entered the Cavan militia (4th battalion Royal Irish Fusiliers) in 1862, and was made a major in 1875. He became a colonel in 1886 and was in command of the battalion from 1891 to 1893.[2] In March 1893, Saunderson was one of the signatories of the manifesto of the Ulster Defence Union, launched to organise resistance to the Second Home Rule Bill of 1893.[5]
Rosa Sarah Saunderson (1867–1952), who married Maj. Henry Nugent Head, a son of Henry Haswell Head, in 1892.[10][11]
Somerset Francis Saunderson (1868–1927), who married Marie Satterfield, a daughter of John M. Satterfield, and former wife of Count Franz Joseph von Larisch.[12][13]
In his private life, Saunderson was well known as a keen yachtsman, and his character was deeply marked by stern religious feeling.[2] He was a devout EvangelicalAnglican.
He died of pneumonia in 1906.[4] In 1910, a statue, erected by public subscription, was unveiled at Portadown.
Through his daughter Rosa, he was a grandfather of Col. Henry William Nugent Head (1898–1964), who married Ruth M. Kresge, daughter of American chain store executive Sebastian S. Kresge. A prominent sportsman, in 1928 he hunted with Theodore and Kermit Roosevelt, who was his best man at his wedding to Ruth in 1933.[24]