The family took their name from Domnallan mac Maelbrigdi, fl. c. 9th/10th century.
Their principal home was at Ballydonnellan, County Galway.
An unrelated family of the same name was located in east Ulster.
The Irish annals record the following:
- Flann Óge Ó Domhnalláin, died 1342
- Mael Sechlainn Ó Domhnalláin, died 1375
- Flann Óc mac Séoan Ó Domhnalláin, died 1404
- Cormac Ó Domhnalláin, died 1436
- Ainglioch Ó Dónalláin, fl. mid-14th or mid-15th century
Later bearers of the name
- Nehemiah Donnellan, fl. c. 1560-1609.
- Sir James Donnellan, Lord Chief Justice of the Common Pleas, c. 1588-1665
- Nehemiah Donellan, 1649–1705, lawyer and Chief Baron of the Exchequer
- Pádhraic Ó Domhnalláin, fl. 1830, duelist
- Padraig Ó Domhnallain, born 1884, short-story writer
- Michael Donnellan, 1900–1964, Clann na Talmhan politician
- Pat Donnellan, born 1941, footballer
- Martin Donnellan, born 1948, Garda Síochána recipient of the Scott Medal
- Michael Donnellan (Gaelic footballer), sportsperson
- Surname info, irishtimes.com