The Barrett clan descended from the Normans, and trace their ancestry to a John Baret, a Norman Knight, mentioned in the Domesday book, who went with William the Conqueror to England in 1066 and settled in Pendyne in Wales after the Norman Conquest.
The Barretts then migrated to Ireland with the Norman warlord Strongbow (Richard Le Clare, the 2nd Earl of Pembroke) as hired mercenaries at the end of the twelfth century in the Norman Invasion of Ireland.
There are two Barrett clans in Ireland, the first branch of the clan are the Munster Barretts of County Cork, and the other branch is the Barrett clan of Connacht, most numerous in the Mayo-Galway mountainous areas. The two clans were believed to be unrelated before recent research proved otherwise. The English pipe rolls of the 13th century clearly indicate that the overlords of both the Cork and the Mayo Barretts were the same people, and the records further indicate that both families came from Wales. To this day, the Barretts and the Barrys of Connacht are known as "the Welshmen of Tirawley".
The similarity of the names of the two Barrett clans was thought purely coincidental. The view prevailed that the Barretts of Cork derived their name from the Norman-French "Barratt," while the Barretts of Connacht derived their name from the gaelic name "Bairéad" which means quarrelsome or warlike. Both branches of the Barrett clans were fully assimilated into Irish culture and married into many old Irish families, they are said to have become "more Irish than the Irish themselves". The chief of the Tirawley Barretts was for centuries known by the Gaelic name Mac Baitín, or Mac Wattin, which eventually evolved into Mac Páidín, or Mac Padine. This is the source of the Padden surname found in Mayo today.
The Barretts, along with the Lynotts, are the subject of the old Irish ballad, "The Welshmen of Tirawley".
Castle Barrett (Castel More) is situated on open grassy position south of the town of Mallow. The castle is in ruins, with just a few parts of the walls standing like stone sentinels. The remaining fragments are from the north and east walls. Castle Barrett was built around the 13th century. It was originally known as Castle More or Castlemore. In 1439 it was taken over by the Earl of Desmond. The Barrett family acquired the castle in the 17th century. The castle was damaged in 1645 by Oliver Cromwell's army. After the Battle of the Boyne in 1690, John Barrett who fought on the side of the Jacobites lost to the Williamites. Castle Barrett was destroyed and 12,000 acres of Barrett land was forfeited.
The first of the name in the United states was James Barrett, who landed in Charlestown, Massachusetts in about 1643. His home eventually was Malden, Massachusetts. His son, James, was in a troop of horse in King Philip's War. Another member of the family, James Barrett, was a colonel in the Lexington Alarm in 1775. 
There are numerous suggested origins for the surname Barrett. The chief source is the first name that has the form Berault or Beraud in French, and means "bear power." It is closely associated in derivation with Bernard, meaning "firm bear." It has also been suggested that Barrett and Barneveld, a name found in the Middle Ages, are associated. Barneveld has the significane of "bear-headed." 
Coat of arms
The blazon of the Barrett coat of arms is: Barry of ten per pale argent and gules counterchanged; Crest: A demi-lion rampant sable, ducally crowned per pale argent and gules. In layman's terms, this means that the shield is divided vertically into two halves, both halves striped horizontally red and white, with the colour changing at the mid point; while the crest is a black half-lion crowned with a red-and-white ducal coronet.
The clan motto is: "Frangas non Flectes: virtus probitas", meaning "Unbowed, Unbroken: Honour and Courage"[dubious ].
Notable clan members:
- Electric Scotland
- Celtic Guitar Music Archived 2010-01-04 at the Wayback Machine.
- The Paddens of Mayo
- Celtic Guitar Music Archived 2012-02-18 at the Wayback Machine.
- "The Barrett Clan". Irish Gathering. Retrieved 30 March 2018.
- Go Ireland Genealogy Archived 2012-03-05 at the Wayback Machine.