Mac An Mháighistir
Mac an MHÁIGHISTIR—VII—M'Amaster, M'Evaster, M'Master, MacMaster, Master, Masterson; 'son of the master' (Irish 'máighistir,' Latin 'magister'). This surname seems to have originated in Co. Cavan, but by the end of the 16th century it had spread into Longford and Roscommon. It is now nearly always anglicised Masterson. The Mastersons of Wexford are of English descent.
Alphabetical Index to Irish Surnames