MonahanIrish From Irish Ó Manacháin meaning "descendant of Manacháin". The given name Manacháin meant "little monk", from manach "monk" and a diminutive suffix.
Monday 3Irish Anglicized form of Mac Giolla Eoin. The last part of the surname was mistakenly taken as the Gaelic word for "Monday", Luain.
MonroeScottish Anglicized (typically American) form of Munro. It was borne by the American actress Marilyn Monroe (1926-1962).
MoranIrish Anglicized form of Irish Ó Móráin meaning "descendant of Mórán", a given name meaning "great, large".
MoriartyIrish From Irish Ó Muircheartach meaning "descendant of Muirchertach". This was the surname given by Arthur Conan Doyle to a master criminal in the Sherlock Holmes series.
O'HaraIrish From the Irish Ó hEaghra, which means "descendant of Eaghra", Eaghra being a given name of uncertain origin. Supposedly, the founder of the clan was Eaghra, a 10th-century lord of Luighne. A famous fictional bearer of this surname is Scarlett O'Hara, a character in Margaret Mitchell's Gone With The Wind (1936).
TaggartIrish, Scottish Anglicized form of Irish Mac an tSagairt meaning "son of the priest". This name comes from a time when the rules of priestly celibacy were not strictly enforced.