McDonnell is born on Slate Street in the lower Falls Road of Belfast, Northern Ireland on September 14, 1951 as one of ten children. He attends a nearby Roman Catholic school. He marries Goretti in 1970 and moves into her sister’s house in Lenadoon. There are only two Catholic houses in this predominantly Ulster Protestant housing estate, and their house is attacked on numerous occasions.
McDonnell is arrested in Operation Demetrius and, along with Gerry Adams and others, is interned on the prison ship HMS Maidstone. He is later moved to HM Prison Maze in County Down for several months. Upon release, he joins the Provisional IRA Belfast Brigade. He meets Bobby Sands during the preparation for a firebomb attack on the Balmoral Furnishing Company’s premises in Dunmurry. During the ensuing shoot-out between the IRA and the Royal Ulster Constabulary (RUC) and British Army, both men, along with Séamus Finucane and Seán Lavery, are arrested. McDonnell and the others are sentenced to 14 years in prison for possession of a firearm. None of the men accept the jurisdiction of the court.
McDonnell agrees with the goals of the Irish hunger strike, namely: the right not to wear a prison uniform; the right not to do prison work; the right of free association with other prisoners; the right to organise their own educational and recreational facilities and the right to one visit, one letter and one parcel per week.
Although McDonnell is not involved in the first hunger strike in 1980, he joins Bobby Sands and the others in the second hunger strike the following year. During the strike he fights the general election in the Republic of Ireland, and only narrowly misses election in the Sligo–Leitrim constituency. He goes 61 days without food before dying on July 8, 1981. He has two children. His wife takes an active part in the campaign in support of the hunger strikers.
McDonnell is buried in the grave next to Bobby Sands at Milltown Cemetery in west Belfast. John Joe McGirl, McDonnell’s election agent in Sligo–Leitrim, gives the oration at his funeral. Quoting Patrick Pearse, he states, “He may seem the fool who has given his all, by the wise men of the world; but it was the apparent fools who changed the course of Irish history.”