LAFFERTY/LAVERTY. Lafferty is an exclusively Ulster name and in 1900 was found only in the counties of Donegel, Derry and Tyrone in Ireland. It stems mainly from O'Lafferty or O'Laverty, Galic O'Laithbheartaigh. both names derive from the Gaelic for 'bright ruler' and are Ulster forms of the Connacht name O'Flaverty or O'Flaherty. The 'F'in Ulster was often aspirated or silent, thus O'Flynn becomes O'Lynn.

The O'Laffertys were a Donegal sept, the chief of which was Lord of Aileach, famous for the great stone fortress, the Grainan of Aileach, at the head of Lough Swilly. The first to bear the surname was Murchadh Ua Flaithbheartaigh, also known as Murchadh Gluin Ilair, "of the eagle knee." King Tyrone, died 972, was described by the Four Masters as the "Tainst of Tyrone" a short time before the chieftaincy of Tyrone passed to the O'Neills. The O'Laffertys were driven from Donegal in the thirteenth century and settled near Ardstraw in County Tyrone, their base perhaps marked by the townland of Linslafferty. Monsignor James O'Laverty, 1828-1906, of the Diocese of Down and Connor, was of this sept. He was born at Carraban, County Down. 

From "The book of Ulster Surnames" by Robert Bell.