Mangan, Mongan, ó Mongáin A Gaelic surname, derived from Mong “a head of long and abundant hair”. A sept in north Connacht. A sept of Thomand, there are townlands called Ballymongaun in the Parishes of Cloncrew and Dromcolliher in Limerick, and in the Parish of Kilnamona in Clare. An erenagh sept of the Cineál Eoghain from the Parish of Termonamongan (in which there is a townland called Ballymongan), in Tyrone. In 1890 Mangan was principally found in Dublin, Limerick, Kerry, and Mayo, and the estimated number of bearers was 2,330. In the United States Mangan is the 7,819th most numerous surname with an estimated 2,750 bearers.