Mannion, ó Mainnín, Mannion A Gaelic surname meaning descendant of Mainnín. A sept of Galway who were formerly chiefs of Sodhan, a district nearly co-extensive with the Barony of Tiaquin. In 1890 Mannion was numerous in Galway, it was also to be found in Roscommon. The estimated number of bearers was 4,076 (including variants). In the United States Manion is the 7,077th most numerous surname with an estimated 5,500 bearers.