The surname Mulcahy is of Irish Gaelic origin, being anglicised from "Ó Maolchathaigh". The Ó Maolchathaigh sept most likely originated in County Waterford, and 16th and 17th century records show that the family was most numerous in that county as well as Cork and Tipperary.
Read more about Mulcahy: Etymology, O'More Theory