In Irish Duffy is Ó Dubhthaigh, from dubhthach, meaning "the dark one". Several different families of the name arose separately in different places, the most important being in Donegal, Roscommon and Monaghan.
In Donegal the family were centred on the parish of Templecrone, where they remained powerful churchmen for almost eight hundred years. Here the name has also been anglicised as Dowey or Doohey. The Roscommon family too had a long association with the church, producing a succession of distinguished abbots and bishops. The area around Lissonuffy in the north-east of the county, which is named after them, was the centre of their influence. From this source, the name is now common in north Connacht. The Monaghan O'Duffys were rulers of the area around Clontibret. They too contributed a great deal to the church, with a huge number of parish clergy of the name. They flourished through the centuries, and Duffy is now the single most common name in Co. Monaghan.
In Ulster the name may also be an anglicisation of the Scots Gaelic Mac Dhuibhshith, more usually rendered as McFie, though also sometimes given as McFee, McAfee or Mahaffy.
Muireadach Ó Dubthaigh (1075-1150) of the Roscommon family was the Archbishop of Tuam who commissioned the famous Cross of Cong, now in the National Museum of Ireland. A successor in the Archbishopric, Cadhla Ó Dubthaigh, was Archbishop of Tuam and ambassador to Henry II in 1175.
Sir Charles Gavan Duffy (1816-1903), of the Monaghan Duffys, had a long and distinguished career as a journalist and politician. One of the founders of The Nation newspaper, he was active in the Young Ireland movement, set up the Tenants’ Rights movement and was elected M.P. for New Ross in 1852. Disillusioned at his lack of success, he emigrated to Australia in 1855 and went on to become Premier of Victoria. Among his sons, Sir Frank Gavan Duffy was Chief Justice of Australia from 1931 to 1936 and Charles Gavan Duffy was Clerk of both the House of Representatives and the Senate. Charles’ son George Gavan Duffy returned to Ireland to become active in the nationalist cause. He was one of the signatories to the Treaty in 1921, and became President of the High Court.