Darby
Darby—Darby in Ulster is one of the anglicised forms of O'Dooyearma (O'Duibhdiorma), an old Donegal sept that was located in the district of Bredagh, near Lough Swilly.
O'Dugan, the Topographer, writes of them: "O'Dooyearma of exalted fame" ruled over Bredach and belonged to the Kinel Eoghain. Though the name is otherwise Dermott, etc., in Ulster, the sept has no origin or connection with the McDermott sept in North Connacht. It is also anglicised Dormer in Co. Meath. The Diarmids of N.E. Ulster belong to the clan Campbell, Argyle.
Alphabetical Index of Anglicised Surnames in Ireland
See also Woulfe’s Irish Names and Surnames
… and O'Hart's Irish Pedigrees