Maguire was born in 1934, the son of a violent alcoholic father, James Maguire, who migrated from Glasgow in 1922. His mother, Annie (née McLaughlin), from Edinburgh, was usually the subject of her husband's beatings, which frequently left her "cut, bloodied and bruised". Maguire was the youngest of four children. His only memory of his father taking any notice of him was when "he'd been away and when he came home, he beat me with his belt". His eldest sibling was Eileen, then Kathleen, then James (Jim) who was eight years older than him, the last child. The family had to move house a number of times when all of the rent money was spent on drink. James Maguire was often found unconscious in the street and his son Jim had to drag or carry him back home. Sometimes their father drank Bay Rum, an aftershave lotion. The family was very poor, and the young Maguire had to borrow his brother's old services overcoat to hide the holes in the clothes he wore to school, and he rarely had socks. He "looked up to his long-suffering mum" and saw her as a "model of Christian piety".