Mag Mhuirneacháin
Mag MHUIRNEACHÁIN—IV—M'Gornghane, M'Gornaghan, Magournahan, (Gordon); 'son of Muirneachán' (diminutive of muirneach, loveable, or one of a troop); pronounced Mag Guarnacháin in the spoken language of Co. Mayo. Compare with Mórbhoirneach which is also anglicised Gordon in the same county. Apparently there is some connection.
Alphabetical Index to Irish Surnames