Rechtaid Rígderg son of Lugaid Laigdech, was, according to medieval Irish legend and historical tradition, a High King of Ireland

High King of Ireland

The High Kings of Ireland were sometimes historical and sometimes legendary figures who had, or who are claimed to have had, lordship over the whole of Ireland. Medieval and early modern Irish literature portrays an almost unbroken sequence of High Kings, ruling from Tara over a hierarchy of...

. He took power after killing Macha Mong Ruad, daughter of his father's killer, Áed Rúad

Áed Ruad

Áed Rúad, son of Badarn, Díthorba, son of Deman, and Cimbáeth, son of Fintan, three grandsons of Airgetmar, were, according to medieval Irish legend and historical tradition, High Kings of Ireland who ruled in rotation, seven years at a time. They each ruled for three seven-year stints...

. He ruled for twenty years, until he was killed by Úgaine Mór

Úgaine Mor

Úgaine Mór , son of Eochu Buadach, son of Dui Ladrach, was, according to medieval Irish legend and historical tradition, a High King of Ireland. He was the foster-son of Cimbáeth and Macha Mong Ruad, and took power by killing his predecessor, and his foster-mother's killer, Rechtaid Rígderg...

, foster-son of Macha and her husband Cimbáeth. The Lebor Gabála Érenn

Lebor Gabála Érenn

Lebor Gabála Érenn is the Middle Irish title of a loose collection of poems and prose narratives recounting the mythical origins and history of the Irish from the creation of the world down to the Middle Ages...

synchronises his reign to that of Ptolemy I Soter

Ptolemy I Soter

Ptolemy I Soter I , also known as Ptolemy Lagides, c. 367 BC – c. 283 BC, was a Macedonian general under Alexander the Great, who became ruler of Egypt and founder of both the Ptolemaic Kingdom and the Ptolemaic Dynasty...

 (323-283 BC). The chronology of Geoffrey Keating

Geoffrey Keating

Seathrún Céitinn, known in English as Geoffrey Keating, was a 17th century Irish Roman Catholic priest, poet and historian. He was born in County Tipperary c. 1569, and died c. 1644...

's Foras Feasa ar Éirinn dates his reign to 461-441 BC, the Annals of the Four Masters

Annals of the Four Masters

The Annals of the Kingdom of Ireland or the Annals of the Four Masters are a chronicle of medieval Irish history...

to 654-634 BC.

If a son of the legendary 2nd century AD Lugaid Loígde, Rechtaid Rígderg appears to have been misplaced chronologically by the later medieval synchronists.