Farrell is born into a middle-class family with no link to militant Irish republicanism other than a grandfather who had been interned during the Irish War for Independence. She is educated at Rathmore Grammar School, Belfast. At the age of 14 she is recruited into the Provisional IRA by Bobby Storey. After leaving school at the age of 18, she is hired as a clerical worker for an insurance broker’s office.
On March 1, 1976, the British government revokes Special Category Status for prisoners convicted from this date under anti-terrorism legislation. In response, the IRA instigates a wave of bombings and shootings across Northern Ireland and younger members such as Farrell are asked to participate. On April 5, 1976, along with Kieran Doherty and Sean McDermott, she attempts to plant a bomb at the Conway Hotel in Dunmurry, as that hotel had often been used by British soldiers on temporary duty in Northern Ireland. She is arrested by Royal Ulster Constabulary (RUC) officers within an hour of planting the bomb. McDermott, her boyfriend, is shot dead by a RUC reservist at a nearby housing estate.
At her trial, Farrell refuses to recognise the court as it is an institution of the British state. She is sentenced to fourteen years in prison for explosives offences, firearms offences, and belonging to an illegal organisation.
When Farrell arrives at Armagh Prison, she refuses to wear a prison uniform in protest at the designation of republican prisoners as criminals and becomes the official Officer Commanding of the female IRA prisoners. After 13 months, she, along with Mary Doyle and Mairead Nugent, begin a hunger strike to coincide with the one already taking place in Long Kesh Prison. It ends on December 19, a day after the men’s strike. In March 1981 the prisoners’ rights campaign is focused on the hunger strike being undertaken by Bobby Sands, leader of IRA prisoners in the H-Blocks. She was one of the H-Block/Armagh prisoners to stand for election in the Republic of Ireland in the 1981 Irish general election, standing in Cork North-Central and polling 2,751 votes (6.05%).
Upon her release from prison in October 1986, Farrell enrolls at Queen’s University, Belfast for a course in Political Science and Economics. However, she drops out to re-engage in IRA activity. The IRA sends her with Seán Savage and Daniel McCann to the British overseas territory of Gibraltar to plant a car bomb in a heavily populated town area. The target is the band and guard of the 1st Battalion of the Royal Anglian Regiment during a weekly ceremonial changing of the guard in front of Governors’ residence, on March 8, 1988. According to interrogated IRA members, Gibraltar is selected as a target because it is a British possession that is in dispute, and that it is an area with lighter security measures than had become endemic at British military installations elsewhere due to the IRA’s campaign.
The British Government’s domestic intelligence service MI5 becomes aware of their plan and a detachment from the British Army is specifically deployed to Gibraltar to intercept the IRA team and prevent the attack. Farrell, Savage and McCann are confronted by plainclothes soldiers from the Special Air Service regiment while they are engaged in a reconnaissance in Gibraltar pending the delivery of the car bomb. Farrell is shot three times in the back and once in the face. Her two accomplices are also killed in an operation code-named Operation Flavius by the British Government. Some witnesses to the shooting state that Farrell and McCann were shot while attempting to surrender, and while lying wounded on the ground. The three IRA members are all found afterwards to be unarmed.
Keys to a hire car found in Farrell’s handbag lead the Spanish Police to the discovery across the border in Spain of five packages totaling 84 kg of Semtex explosive in a car which the IRA team intended to subsequently drive into Gibraltar for the attack. These packages have four separate detonators attached. Around this is packed 200 rounds of ammunition as shrapnel. There are two timers but they were not primed or connected.