The McDonnells of Antrim are descended from John Mor MacDonald, chief of the Clan MacDonald of Dunnyveg. John Mor MacDonald was the second son of Good John of Islay, Lord of the Isles, 6th chief of Clan Donald. Through John of Islays second marriage to Princess Margaret Stewart, daughter of King Robert II of Scotland.
John Mor MacDonald himself married Margery Bisset of the Glens of Antrim. This acquired him vast lands in Ireland added to those he already possessed including Dunnyvaig Castle on the Isle of Islay and lands in Kintyre, Scotland. John Mor thus became the chief of the Clan Mcdonnell of Antrim where he was seated at Dunluce Castle.
The MacDonald clans of Antrim and Dunnyveg under the same chief have similar histories however one being in Ireland and the other in Scotland, respectively, they are often treated as two separate clans, but they are two branches of Clan Donald.
John Mor was assassinated by James Campbell in 1427. His son Donald Balloch MacDonald, the second chief, led the clan when they fought and won at the first Battle of Inverlochy (1431). This was in support of their cousin Alexander of Islay, Earl of Ross, 3rd Lord of the Isles and 8th chief of Clan Donald. The MacDonalds were supported by the Clan Cameron. They fought against the Royal forces of the Earl of Mar who was supported by the Clan MacKintosh.
The third Chief, Sir John Mor, with his heir John Cathanach and three grandsons, were apprehended through the treachery of the Macdonald of Ardnamurchan and were executed in Edinburgh for treason. However MacIian of Ardnamurchan who had also betrayed Alexander MacDonald of Lochalsh and was himself killed in 1518 by those who he had betrayed.
On the death of James MacDonald, the sixth chief of the Clan MacDonald of Dunnyveg and Antrim, the Antrim Glens were seized by one of his younger brothers, Sorley Boy MacDonnell (Somhairle Buidh:Somhairle of the yellow hair).
In 1565 under Sorley Boy MacDonnell the Clan MacDonald of Antrim and Dunnyveg fought the Battle of Glentasie against Shane O'Neill in Ireland. Sorley Buy swore allegiance to James IV of Scotland and his son Ranald was made Randal MacDonnell, 1st Earl of Antrim by Queen Elizabeth. Randal, 1st Earl of Antrim was succeeded by his son Randal MacDonnell, 1st Marquess of Antrim.
Much quarreling took place between Angus MacDonald, 8th of Dunnyvaig and his eldest son, Sir James MacDonald this was largely due to the intrigues of the Clan Campbell. Sir James MacDonald led the clan who fought and won at the Battle of Gruinneart Strand on the Isle of Islay in 1595 against an invasion force of the Clan MacLean who were led by their notorious chief Sir Lachlan MacLean of Duart Castle who was killed.
The current chief of the Clan MacDonnell of Antrim is Alexander Randal Mark McDonnell, 14th Earl of Antrim and Viscount Dunluce.