Stagg is the seventh child in a family of thirteen children. His father, Henry, and his uncle had both fought in the Irish War of Independence and Irish Civil War. His brother, Emmet Stagg, becomes a Labour Party politician and a Teachta Dála (TD) for Kildare North. He is educated to primary level at Newbrook Primary School and at CBS Ballinrobe to secondary level. After finishing his schooling, he works as an assistant gamekeeper with his uncle prior to emigrating to England in search of work. Once in England, he gains employment as a bus conductor in North London and later becomes a bus driver. While in England he meets and marries fellow Mayo native, Bridie Armstrong from Carnacon in 1970.
In April 1973, Stagg is arrested with six others alleged to comprise an IRA unit planning bombing attacks in Coventry. He is tried at Birmingham Crown Court. The jury finds three of the seven not guilty. The remaining four are all found guilty of criminal damage and conspiracy to commit arson. Stagg and English-born priest, Father Patrick Fell, are found to be the unit’s commanding officers. Stagg is given a ten-year sentence and Fell is given twelve years. Thomas Gerald Rush is given seven years and Anthony Roland Lynch, who is also found guilty of possessing articles with intent to destroy property, namely nitric acid, balloons, wax and sodium chlorate, is given ten years.
Stagg is initially sent to the top security Albany Prison on the Isle of Wight. In March 1974, having been moved to Parkhurst Prison, he and fellow Mayo man Michael Gaughan join a hunger strike begun by the sisters Marian Price and Dolours Price, Hugh Feeney and Gerry Kelly.
Following the hunger strike that results in the death of Michael Gaughan, the Price sisters, Feeney and Kelly are granted repatriation to Ireland. Stagg is denied repatriation and is transferred to Long Lartin Prison. During his time there he is subject to solitary confinement for refusing to do prison work and is also subjected, along with his wife and sisters during visits, to humiliating body searches. In protest against this, he begins a second hunger strike that lasts for thirty-four days. This ends when the prison governor agrees to an end of the strip-searches on Stagg and his visitors. He is bed-ridden for the rest of his incarceration in Long Lartin, due to a kidney complaint.
In 1975 Stagg is transferred to Wakefield Prison, where it is demanded that he again do prison work. He refuses and is placed in solitary confinement. On December 14, 1975, he embarks on a hunger strike in Wakefield, along with a number of other republican prisoners, after being refused repatriation to Ireland during the IRA/British truce. His demands are an end to solitary confinement, no prison work and repatriation to prison in Ireland. The British government refuses to meet any of these demands and Stagg dies on February 12, 1976 after 62 days on hunger strike.
Stagg’s burial causes considerable controversy. Republicans and two of his brothers seek to have him buried in the republican plot in Ballina beside the grave of Michael Gaughan, in accordance with his wishes. His widow, his brother Emmet Stagg and the Irish government wish to have him buried in the family plot in the same cemetery and to avoid republican involvement in the funeral.
In order to prevent the body from being disinterred and reburied by republicans, the grave is covered with concrete. Local Gardaí keep an armed guard by the grave for six months. However, unknown to them, the plot beside the grave is available for purchase. Stagg’s brother George purchases the plot and places a headstone over it, with it declaring that the “pro-British Irish government” had stolen Frank’s body. In November 1977, a group of republicans dig down into the plot that George had purchased, then dig sideways and recover Stagg’s coffin from the adjacent plot under cover of darkness, before reburying it in the republican plot beside the body of Michael Gaughan. The Republicans hold their own version of a funeral ceremony before disappearing back into the night.
Following the final burial, an anonymous letter is sent to Taoiseach Liam Cosgrave, Minister for Justice Patrick Cooney, Minister for Post and Telegraphs Conor Cruise O’Brien and Minister for Foreign Affairs Garret FitzGerald, informing them each that they have been “marked out for assassination” because of their government’s involvement with Stagg’s burials. Stagg’s widow Bridie and his brother Emmett are reported to be intimidated by members of the Provisional IRA due to their opposition to his burial in a Republican plot.
The IRA swears revenge over Stagg’s death, warning the British public it is going to attack indiscriminately. They explode about 13 bombs throughout England within a month after his death.