William O'Sullivan was the eldest son of Daniel O'Sullivan, a carpenter, and Ellen nee Carey. He was born at St George's Quay in Cork City but the family then moved to Winter's Hil where they remained for the rest of his parents' lives. He had 3 younger sisters: Margaret, Mary and Eileen, and a brother, John Joseph, who became a Christian Brother in Dublin but died of TB at the age of 26.
William followed in his father's footsteps and became a carpenter. Sometime in the early 1920s he moved to Liverpool, where he met and married another Irish exile, Mary Margaret Rowan, from Limerick. He became a school woodwork teacher and had 4 children with Mary before her early death from bowel cancer in 1934.
He took Finbar as his confirmation name and used that as his middle name for the rest of his life.
He died in Liverpool in 1985, a few weeks before his 85th birthday.