A prominent branch of the Uíbh Eachach Cobha, the Magennises would become chiefs of the territory of Iveagh, which by the 16th century comprised over half of modern County Down.

[1][2] They ruled the sub-territory of Clann Aodha (Clan Hugh), however by the 12th-century had replaced the Ui hAitidhe as the chiefs of Iveagh, with Rathfriland as their base.

[2] The Magennises are also mentioned in letters by King Edward II, where they are titled Dux Hibernicorum de Ouehagh, meaning "chief of the Irish of Iveagh".

[citation needed] After the subsequent collapse of the earldom, the Magennises by the 15th century had expanded Iveagh all the way east to Dundrum Castle, where County Down meets the Irish Sea.

[3] By 1500 there were twelve branches of the Magennis clan,[citation needed] the most prominent being: Castlewellan, Corgary, Kilwarlin, and Rathfriland, the rivalry between whom threatened the cohesion of Iveagh.

One chief, "Arthur Guinez", was on the losing side in the Battle of Bellahoe while invading County Meath in 1539.

[7][3] During this war, Baron Mountjoy, the Lord Deputy of Ireland, ravaged Iveagh to the point where Art Roe Magennis submitted to prevent the extermination of his people, and as such was promised he could keep his lands.

[citation needed] Following this and the Cromwellian conquest of Ireland, the Magennises of Iveagh lost out significantly with all their lands but those at Tollymore being forfeited, with four of the leading Magennis freeholders transplanted to the province of Connacht.

[3] The king also sought to have the 20,161-acre ancestral estate of Arthur Magennis, 3rd Viscount Iveagh, restored, but this was prevented by local landowners.