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Leath Cuinn (Conn's Half) andLeath Moga (Mug's half) are legendary ancient divisions of Ireland, respectively north and south of a line corresponding to theEsker Riada running east–west fromDublin Bay toGalway Bay. The eponymous Conn and Mug wereConn Cétchathach (Conn of the Hundred Battles) and Éogan MórMug Nuadat (the Servant ofNuada), whose armies in 123 AD fought the battle ofMag Lena (the Plain of Lena, in what is nowCounty Offaly betweenTullamore andDurrow).^[1]

At Mag Lena, the army of Conn, theHigh King of Ireland, lost to that of Mug Nuadat, theking of Munster, to whom Conn was thus forced to cede the southern half of Ireland. Thereafter theprovinces of Ireland were grouped as follows:

• Leath Cuinn (Conn's Half, the north) comprisedConnacht,Ulster andMeath;
• Leath Moga (Mug Nuadat's Half, the south) comprisedMunster (includingOsraighe) andLeinster.

To solidify the arrangement, Conn's daughterSadb was married toAilill Aulom, son of Mug Nuadat. Their son was anotherÉogan Mór, founder of theEóganachta dynasty which ruled Munster.

Conn was the ancestor of the dynasties of theConnachta (named after him and later eponymous overlords of Connacht) and their northern offshoots theUí Néill (of whose descendants theNorthern Uí Néill drove theUlaid out of west Ulster, while theSouthern Uí Néill took most of Meath).

The Eóganachta's control ofLeath Moga was largely confined to Munster. David Sproule of theDublin Institute for Advanced Studies suggests the namesLeath Cuinn andLeath Moga originally had their literal meaning "Head Half" and "Slave Half", with the figures of Conn and Mug Nuadat created centuries later asetiological myth, elaborated by the Eóganachta to bolster their territorial claims.^[2] Their historic right to rule Leinster as part ofLeath Moga was disputed by the Southern Uí Néill, while Osraighe was formally ceded toLeath Cuinn in 859.^[3]

TheSynod of Ráth Breasail in 1111 AD createdterritorial dioceses in Ireland, divided into twoecclesiastical provinces, with archbishopsin Armagh andin Cashel, respectively corresponding toLeath Cuinn andLeath Moga. This was altered when the 1152Synod of Kells separated the provinces ofTuam andDublin from Armagh and Cashel respectively.

SeeAnnals of Inisfallen (AI)

• AI929.2 Repose of Tuathal, learned bishop of Leth Cuinn.

• Aubrey Gwynn,The Irish Church in the Eleventh and Twelfth Centuries, Gerard O'Brien (ed.) (Four Courts Press, 1992).
• Seaán Mór Ó Dubhagáin andGiolla-Na-Naomh Ó Huidhrín,Topographical Poems ,James Carney (ed.) (Dublin Institute for Advanced Studies, 1943).

• Irish legends
• Ancient peoples
• Historical geography of Ireland

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