Leath Cuinn (Conn's Half) and Leath Moga (Mugh's half) refers to a legendary ancient division of Ireland.
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Geographical extent
Leath Cuinn was the island north of the Esker Riada (east-west drumlin belt from Dublin to Galway Bay). Conn Cétchathach, for whom this division is named, was a retrospective ancestor of the Connachta and Uí Néill dynasties.
Legendary origin of the term
As a result of the battle of Maigh Nuadad in 123 A.D., where he was defeated by Eoghan Mor (also known as Mug Nuadat), Conn was forced by Eoghan to accede to the division of Ireland into two halves:
Ramifications
To solidify the arrangement, Conn's daughter, Sadbh was married to Ailill Aulom, son of Eoghan.
The Eoganacht dynasty in Munster claimed, as descendants of Oiloill Olum, the historical right as overkings of Leath Moga (i.e. overlordship of Leinster as well as Munster) in the early medieval age, a claim disputed by the Ui Neill.
In the twelfth century, the north-south division of Ireland was used as a basis for the new division of dioceses in Ireland at the Synod of Ráth Breasail.