Ó Monacháin, Ailbhe (1889–1967), illustrator and Irish-language activist, was born 26 January 1889 in Belfast and christened Alfred Monahan, youngest of four sons and two daughters of Wexford-born Robert Joseph Monahan, sawyer, and Johanna Monahan (née Nolan). He was educated in Belfast by the Christian Brothers, where he developed a love of the Irish language. On leaving secondary school he joined Conradh na Gaeilge and spent some time on the Belfast district committee. He taught Irish to the junior classes while continuing his own studies of the language with the senior classes. Eventually he was awarded a higher teaching certificate in Irish. He was the only member of his family to speak Irish fluently, and he filled out the census form of 2 April 1911 in Irish and signed his name as the head of the family.
He served an apprenticeship as a commercial artist with the Belfast printers David Allen & Sons, but did not practise much after he joined the Irish Volunteers, recently formed in Belfast. As an assistant to Liam Mellows (qv) he was involved in the campaign against recruitment to the British army (1915–16) and extended the campaign into Connemara, where his knowledge of the Irish language proved most useful. He took part in the 1916 rising in Athenry, Co. Galway, and spent four months on the run in Co. Clare with Mellows and Proinsias Ó hEidhinn, later helping Mellows escape to America. In Cork (1917) he tried to energise the volunteers while he worked on a farm and taught Irish under an assumed name (Tadhg Mac Suibhne). He later boasted that he was the only ‘foreigner’ ever to teach Irish in Cork. He was imprisoned several times for his activities during the war of independence and was on the anti-treaty side during the civil war. From 1924 to 1927 he was an inspector for Conradh na Gaeilge, and as an organiser in south Ulster was involved in efforts (1925) to have the Irish College of Glangevlin, Co. Cavan, officially recognised by the Gaeltacht commission as a breac-ghaeltacht area.
Ó Monacháin taught Irish and later arts and crafts in Balbriggan and Swords, Co. Dublin, for the vocational education committee. He worked with Brian O'Higgins (qv), a Dublin printer, illustrating and writing verses for cards and calendars. Some of his illustrations appear in the Wolfe Tone Annual (1946), also published by O'Higgins. His stories and illustrations about the war of independence, titled ‘Scéalta fíora Óglach’ were published in An tUltach. He wrote and translated a number of engaging children's books with beautiful illustrations, published by An Gúm; among them were Patairín agus Pataru´n (1938); Sceilíní caoine ar chroidhe cogaidh (1939), translated from German; and Ainmhithe allta na hÉireann (1969). He was president of the Oireachtas for one year (1961). He married Péig Ní Bhuachalla; having no family of their own, they adopted a German girl, Wilhelmina Putz, during the second world war. He died 30 July 1967 at his home in Swords, Co. Dublin.