Thomas O'Herlahy (O'Herlihy, O'Hiarlaithe) (died 1579) was the Catholic Bishop of Ross, Ireland (Rosscarbery).
His kinsman, the future Archbishop of Cashel Dermot O'Hurley, one of the most celebrated of the Irish Catholic Martyrs, is believed to have received his early education at a Cathedral school overseen by Bishop Thomas O'Herlahy, to whom he may have been given in fosterage, in the monastery founded by Saint Ailbe of Emly.
Simultaneously with Archbishop Richard Creagh, he was confined until released after about three years and seven months on the security of Cormac Laidir, Chief of the Name of Clan MacCarthy of Muskerry.
The Bishop intended to retire to Flanders, but ill health contracted in prison induced him to return to Ireland.
He made a visitation of his diocese yearly, and on great festivals officiated and preached in a neighbouring church.