A native of Draperstown, County Londonderry, Donnelly was born on 20 July 1878, the son of Patrick Donnelly, a spirit merchant and farmer, and his wife, Sarah Convery.[1] He was educated privately and at Queen's College Belfast. He served his solicitor's apprenticeship with Michael McCartan, the MP for South Down. He qualified in 1905.[2]
Donnelly was living in Camlough at the time of the 1901 census.[3] In 1909 in Belfast, he married Susanna Rogers, of Draperstown.[4]
According to an obituary, he was the second last of the surviving MPs who had voted against the Treaty of Versailles. He died at his residence at Windsor Hill, Newry.[6]