O'Connell was born near Knock, County Mayo as Thomas J. Connell, the second child of Mary Biesty and Tom Connell. As part of the Gaelic revival, his family adopted the prefix "O" to their surname in the early 20th Century. His father was a farmer who was politically active and a community leader. Family lore held that his father had been a Fenian who had taken part in the Fenian Rising of 1867. He was also an activist in the Land League and a member of the United Irish League. One of Thomas's aunts, Mary O'Connell, had been one of the first witnesses of the Marian Apparition at Knock in 1879. Perhaps unsurprisingly, this may have influenced the fact that Thomas was a devout Catholic throughout his life.