(O) Farrells/Ferrells and others worldwide often ponder their Irish roots. This is currently the most comprehensive attempt to explore the origins of one of the largest branches of the Farrells/Ferrells. It includes: 1,400 years of Celtic roots in northwest Ireland, Gaelic ancestry linked to St Colum Cille (St Columba) from c.AD 655, 400-year-old associations with the Ulster Plantation, and worldwide migration. Those wishing to explore their own Irish family history and genealogy may use the methodology adopted by the author as a template for their own research. Almost 1,000 references are detailed, representing an invaluable resource to all those researching their Irish and Ulster roots. The benefits of DNA testing in family history and genealogy are outlined, and the results of the Donegal Farrell/Ferrell DNA research are analysed. Extensive genealogies of Ulster Farrells/Ferrells and associated families from the sixteenth to twenty-first centuries have been compiled, and this database will assist others research their roots in Donegal, Ulster, and Ireland.