If you have one of the following names--or one of these names prefixed by O’--then you are a member of an Irish Clan – The Farrell Clan:
Farrell, Far(r)el(l)(y), (Farley), Ferrall, Farrill, Ferrall, Fer(r)al(l)(y), Ferrell, Ferrill, Frawley.
The Farrell variation is the most common. Before these names, the O' prefix is a variant of the Gaelic Ui or Ua, meaning simply ‘grandson.’  Today, it includes any descendant.
Irish surnames are among the oldest in the Western world. The earliest fixed surname in the Western world is Ó Cleirigh, which can also be spelled O'Cleary. It dates back to 916 AD, making it over 1,100 years old. Farrell came into existence in 1014 with the death of Fhearghaíll (Farrell), the clan chieftain, at the Battle of Clontarf.
The clan was then called the Conmaicne. They took their name from a mythical ancestor known as Conmac. Conmac was, in turn, said to be descended from Fergus mac Roich and Queen Méibh (Maeve) of Connacht. The word Conmacne means "progeny of Conmac." The clan soon took the name Fhearghaíll (Farrell) to honor the slain chieftain. Today, the name of County Longford comes from the Clan name, the County's full Gaelic name being Longphuirt Uí Fhearghaíll, "O'Farrell's fortress."
There are two branches to the Conmaicne:
Conmhaicne Rein, or "lower Conmaicne," of south County Leitrim into County Longford.
Conmhaicne Angaile (Annaly), or "upper Conmaicne," now county Longford.
The Diocese of Ardagh was established in 1111 as the see for east Connacht. Fourteen years later, at the Synod of Kells-Mellifont, its area was reduced to the territory of the Conmaicne Rein and Conmaicne Angalie, south county Leitrim, and all county Longford. The diocese was commonly called "the Diocese of the Conmaicne."

Some information has been gleaned from “Place Names of County Longford,” Rev. Joseph MacGivney, 1908