MacManus is the anglicisation of the Irish Mac Maghnuis, from the popular Norse personal name Magnus, from the Latin magnus, "great". Although the Viking settlers are responsible for the introduction of Magnus as a personal name, the surname it gave rise to is entirely Irish. It came into being in two distinct areas, in Co. Roscommon, where the family claim descent from Maghnuis, son of the twelfth-century High King, Turlough O'Connor, and in Co. Fermanagh, where the original ancestor was Maghnuis Maguire, son of the chieftain Donn Mor Maguire. In Fermanagh they were second in power only to the Maguires themselves, and from their base on the island of Ballymaguire (now Belleisle) on Lough Erne controlled the shipping and fishing on the lake. Cathal Óg MacManus (1439-1498), chief of the name, dean of Lough Erne and vicar-general of the diocese of Clogher, was responsible for the compilation of the Annals of Ulster. Today the surname is most common by far in its original homelands, and especially in Co. Fermanagh. Other variants of the name in Ulster include Mayne, Moyne and, in Tyrone, Manasses.
Placenames reflecting the presence of the family are found principally in the areas of their traditional influence in the west and north: Catronmacmanus in Killasser in Co. Mayo, Annaghmacmanus in Loughgall in Armagh, Knockmacmanus in Aghavea in Fermanagh, Lismacmanus in Rathcline in Longford.
Anna MacManus née Johnston (1866-1902) wrote under the name Ethna Carbery. A prolific and popular poet, she founded The Northern Patriot and was a passionate supporter of the original Sinn Féin.
Francis MacManus (1909-1965), born in Kilkenny, published 11 novels between 1937 and 1959, as well as numerous short stories, biographies, essays and travel books. he is best remembered for his finest book, The Fire in the Dust (1950).
Terence Bellew MacManus (1823-1860), from Fermanagh, was transported for his part in the Young Ireland rising and escaped to America from Tasmania. His funeral, in Dublin, was the occasion of one of the largest Fenian demonstrations.
Declan MacManus (1950 - ) is better known as Elvis Costello. The son of Irish parents who emigrated to London, he is one of the most original and influential songwriters of his generation.