This Gaelic-Irish surname is written as "Ua Flaithbertach" (nominative) or "Ua Flaithbertaig" (genitive) in Old Irish and Middle Irish texts. In Modern Irish the surname is now generally spelt as Ó Flatharta.

The surname is commonly translated as "bright ruler" or more correctly "bright prince", flaith originally meaning prince in Irish. "O" or Ó comes from Ua, designating "grandson" or "descendant" of a (major) clan member. The prefix is often anglicised to O', using an apostrophe instead of the Irish síneadh fada: "'".

Maigh Seóla was the earliest O'Flaherty domain, to the east of Lough Corrib in the kingdom of Connacht, the westernmost province of the Island of Ireland (Irish: Éire).

The Ó Flaithbertaighs are a branch of the Muintir Murchada dynasty, named after Murchadh mac Maenach (died 891), King of the Uí Briúin Seóla. Murchadh is one of the earliest attested kings of his region. The leading family of this dynasty would take the surname Ó Flaithbheartaigh (O'Flaherty) from the 11th century onwards.

In Ulster, the F is aspirated causing the name to be Anglicised as Laverty etc.

The Uí Briúin Seóla was one of the major branches of the powerful Uí Briúin dynasty, which had become the dominant force in Connacht by the 8th century. The genealogies list two sons of Murchadh mac Maenach: Urchadh and Urumhain. Urchadh mac Murchadh, King of Maigh Seóla (also listed as king of Iarthair Connacht, died 945, in the 14th century Book of Ballymote) was father of Bé Binn inion Urchadh, Princess of the Uí Briúin Seóla and Queen of Thomond (fl. early 10th century). Bé Binn married Cennétig mac Lorcáin of Thomond to produce a son who would become the High King of Ireland (Irish: Ard-Rí na hÉireann): Brian Bóruma mac Cennétig, known in English as Brian Boru (c. 941–23 April 1014). Brian broke the near monopoly of the Uí Néill over the High Kingship of Ireland and fought to unite Ireland as a people under one, native king. His father, Cennétig mac Lorcáin of Thomond, was one of the principal leaders of the resistance to the Danish Viking incursions. Cennétig had several wives and children but positively assigned Bé Binn as the mother of Brian Bóruma.

Bé Binn's sister Creassa inion Urchadh was a wife of King Tadg mac Cathail of Connacht, while her sister, Caineach inion Urchadh, married the ancestor of the Clann Coscraig sept of the Uí Briúin Seóla. Her brother, Donnchadh mac Urchadh succeeded their father as King of Maigh Seóla (943–959).

Like the Uí Briúin Seóla, the Uí Briúin Ai were a major branch of the Uí Briúin dynasty, from whom the high medieval Ua Conchobair (O'Connor) kings of Connacht, including the last high king of Ireland, Ruaidrí Ua Conchobair, were descended.

The Ó Flaithbertaigh family line and kingship can be traced back to Brión mac Echach Muigmedóin, King of Connacht, who was the half-brother of Niall of the Nine Hostages. Their father, Eochaid Muigh Meadhoin mac Muiredach, was, according to legend, also 122nd High King of Ireland and was a direct descendant of Galamh Milesius (c. 1763 BCE – c. 1699 BCE). Galamh Milesius's bloodline produced kings in succession for over 3,000 years. Kingship was passed on to direct descendants only starting with Érimón mac Míl Espáine, 2nd High King of Ireland son of Galamh Milisius.

In Connacht the first to bear the name was Muireadhach ua Flaithbheartach, king of Maigh Seóla, who died in 1034. He is listed as having three sons – Ruaidhrí of Lough Cimbe, Donagh Aluinn and Aedh. From Ruaidhrí and Donagh would descend the O'Flahertys of East and West Connemara.

In the Annals of the Four Masters, Rúaidhri Ua Flaithbheartaigh, King of Iar Connacht, is described as dying at the battle of Glen Patrick in 1061 (Book of Ballymote: died 1062). The Annals state:

The following year records that Tadhg, son of Aedh Ua Conchobhair (O'Connor), was "slain by the son of Aedh, son of Ruaidhri (ie O'Flahertys), and the people of Iar Connacht." From this point on, the family were forced into Iar Connacht, as the Kings of Connacht took the original Ua Flaithbheartaigh homeland for themselves.

Aedh Ua Flaithbheartaigh, (King of Iar Connacht, died 1079) was the third bearer of the surname Ua Flaithbheartaigh to rule over the Muintir Murchada. Aedh was killed in 1079 by Ruaidrí na Saide Buide (Ruaidrí Ua Conchobair, anglicised Roderic O'Connor, died 1118), who in turn was blinded by Flaithbertaigh Ua Flaithbertaigh (Flaherty O'Flaherty), King of Iar Connacht, in 1092.

Crichaireacht cinedach nduchasa Muintiri Murchada is a tract listing the main families and their estates of the Muintir Murchada in the reign of its lord, Flaithbertaigh Ua Flaithbertaigh, who was king of Iar Connacht from 1091 until his death in 1098. He seized the kingship of Connacht in 1092 by blinding the Godfather of his children, King Ruaidrí na Saide Buide. He was killed by Ruaidrí's sons in 1098.

The subsequent king of Iar Connacht, Muireadhach Ua Flaithbheartaigh (died 1121), and his descendants, remained loyal to the Ó Conchobair's.

Before the close of the 13th century, the Ó Flaithbertaigh clan became masters of the entire territory of Iar Connacht, extending from the western banks of Lough Corrib to the shores of the Atlantic. In the 1230s, the Normans invaded Ireland and took Galway. Native Irish were forbidden from entering the city freely. In the 16th century, the Ó Flaithbertaigh built a tower house called Aughnanure Castle, which was featured on an Irish stamp and is now a national historic property and tourist destination.

The clan motto is Fortuna Favet Fortibus, or "Fortune Favours the Brave" which may have been inspired by the same line in Virgil's Aeneid. The Ó Flaithbertaigh coat of arms depicts "two red lizards or dragons rampant combatant, supporting a red dexter hand, couped at the wrists, in base a black boat with eight oars". The two red lizards or dragons are often confused with the lions of the English heraldic tradition. Often a grey or green salamander is depicted on a black or grey helmet above the coat of arms.

However, there is little known of Irish heraldic tradition as compared to that of the English although Irish use of distinctive flags and banners in battle is recorded in Irish annals such as the Annals of the Four Masters.