Mac Aonghusa, Criostóir (1905–91), teacher, author, critic, and campaigner for the Irish language, was born 13 December 1905 in Gort an Ghabhainn, Banagher, King's Co. (Offaly), son of Francis McGuiness (surname thus on his birth certificate), farmer, and his wife, Rosanna (née Egan). He was educated at La Sainte Foi, Banagher, Reachra national school, Shannonbridge, and Naas CBS. He qualified as a primary school teacher from De La Salle Training College, Waterford, in 1926 and had the distinction of being the first person to sit all exams through the medium of Irish. He graduated BA at UCG in 1933 and his further education included a diploma in Spanish literature from Barcelona University and an MA on the Irish scholar Tomás Ó Máille (qv).
His teaching career began in 1926 when he became headmaster on Inis Treabhair, Co. Galway, spending fourteen months there. Afterwards he transferred to Gort Mór, Rosmuc, Co. Galway, where he continued teaching until 1962. He remained an active member of Cumann na Múinteoirí Náisiúnta throughout his life. Between 1962 and 1972 he was employed as an ad-hoc examiner at the civil service commission. An active member of Fianna Fáil, he helped to organise the party in Co. Galway in the 1920s and 1930s, and was elected a member of Galway county council in 1934.
Mac Aonghusa was a prominent advocate of the Irish language and together with his close friend Máirtín Ó Cadhain (qv) and another Connemara schoolteacher, Seosamh Mac Mathúna, founded Cumann na Gaeltachta to agitate for the civil rights of the gaeltacht communities and of Irish-speakers in general. He was one of the main campaigners for the establishment of the Rath Carn gaeltacht in Co. Meath and formed part of the delegation that met Éamon de Valera (qv) on 11 November 1932 and received from him a promise to provide land in Co. Meath for that purpose. The gaeltacht was established in 1935. Mac Aonghusa continued to support the project throughout his life and was involved in further campaigns relating to the area, including the recognition of Rath Carn's gaeltacht status. He was also an active member of Conradh na Gaeilge in the 1940s and later involved in the campaign for the establishment of an Irish-language television broadcasting service.
Mac Aonghusa was a prolific writer and began publishing short stories and articles from 1926 onwards. His contributions appeared in An tÉireannach, An Phoblacht, the Irish Tribune, An Stoc, and Ar Aghaidh. From 1948, he was a regular contributor to Feasta and his essays and reviews on Máirtín Ó Cadhain's works appeared in Comhar. He was a member of several literary organisations, including Cumann na Scríbhneoirí and the Galway Literary Society. His first book, An Cladóir agus scéalta eile, appeared in 1952. Between 1963 and 1972 he was a contributor to the Irish Press and worked also as a literary journalist. An essay on Pádraic Ó Conaire (qv) earned him a prize from Acadamh Liteartha na hÉireann.
He spoke a number of European languages including German, Spanish, French, Romanian, and Greek and travelled widely throughout Europe. In the early 1970s he lived in Russia, and a collection of essays entitled Ó Rosmuc go Rostov was published in 1972. For health reasons he lived in Malaga, Spain, from the middle of the 1970s until 1987. While there, RTÉ produced a documentary on his life entitled ‘Ó Ros Muc go Malaga’.
Mac Aonghusa died 9 April 1991 in Portiuncula Hospital, Ballinasloe, Co. Galway, and is interred in Clonmacnoise. In 1930 he married Mairéad Ní Lupain, a nurse from Annaghvane in Connemara, and had four children, Proinsias (1933), Micheál (1937), Róisín (1939), and Máirín (1944). The couple separated in the 1940s. Proinsias followed in his father's footsteps as a writer and journalist and became president of Conradh na Gaeilge and chairman of Bord na Gaeilge.