The Battle of Glentaisie is fought in the north of Ulster on May 2, 1565. The result is a victory for Shane O’Neill over the Clan MacDonald of Dunnyveg. The conflict is a part of the political and military struggle for control of the north of Ireland, involving the English and occasionally the Scots. Although the MacDonalds are a Scottish family, based principally on the island of Islay in the Hebrides, they have long been associated with the Gaelic polity rather than the Kingdom of Scotland.

O’Neill assembles his army for the attack on the MacDonnells at the tower house of Feadan, at Fathom Mountain, near Newry. Traditionally, he “kept Easter” at Feadan, and an assembly of his clan would be unremarked until its size draws notice.

The MacDonnells expect O’Neill’s sizeable army to take some weeks to reach their territory on the Antrim coast. They start gathering as many warriors in the Highlands and Islands as they can muster. However, O’Neill advances with unprecedented speed. Within a week his army arrives at Edenduffcarrick.

Sorley Boy MacDonnell gathers a small holding force at a forward base at Cloughdonaghy. He attempts to stop O’Neill’s advancing army at Knockboy, a wooded pass in the hills above Broughshane. O’Neill sweeps Sorley’s ambush away, takes Cloughdonaghy and sends a force of cavalry to seize James MacDonald of Dunnyveg’s recently constructed Red Bay Castle. With the landing beaches at Waterfoot and Cushendall now controlled by O’Neill’s army, the first several hundred of MacDonnell’s men are compelled to land farther north. They land at Ballycastle beach to await the locally raised MacDonnnell army. Their plan is to await the arrival of their brother Alexander, the MacDonnell seneschal of Cantyre, who is assembling a second army composed from late comers to James of Dunnyveg’s army.

O’Neill does not permit the MacDonnells any time for their reinforcement to arrive. His advance guard continues to press the retreating MacDonnell army, who fall back from the beach through Ballycastle. They are driven past the area of the modern Diamond and well away from the river Tow, denying them the necessary water supply for a sizeable army.

Both armies set up camp for the night. O’Neill’s army occupies land at the centre of modern Ballycastle, between the modern Diamond and the river Tow, giving them full access to water. The combined MacDonnell army of around five hundred men occupy the exposed higher ground at Ramon, the ridge at the head of modern Castle Street, Ballycastle, where the Presbyterian Church now stands, with only a small well for their needs.

At very first light, O’Neill attacks uphill with a sudden onslaught led by his heavy Gallowglass infantry. By forgoing the customary exchange of spears, darts, and arrows by the light infantry kernagh and Scots archers that customarily precede the usually decisive deployment of the Gallowglass corrughadh, or battalions, O’Neill surprises the MacDonnell army. The MacDonnell leaders try to rally their men, but after a violent interlude of some confusion, they break and flee over Knocklayd mountain in the direction of Glenshesk, heading back toward the beach at Cushendun. James of Dunnyveg is seriously wounded in the early fighting and his brother Angus MacDonnell is killed.

The attempt to flee by the old mountain road between Greenan and Ballypatrick Forest in an attempt to reach a possible landing place for their birlinns at Cushendun beach is finally stopped at a hollow at Legacapple. Sorley Boy and James are both taken prisoner. James dies of his wounds two months later at Castle Crocke, near Strabane. Their brother Alistair Og MacDonnell had landed at Rathlin, with the final levies raised in the Highlands and Islands, reputed to be nine hundred men. However, in the immediate aftermath of O’Neill’s decisive victory, Alistair can achieve nothing without a base on the mainland. O’Neill marches on along the north coast to mop up the MacDonnell garrisons and deny Alistair any foothold from which to launch a MacDonnell recovery. Dunseverick and Dunluce fall within a few days, and Alistair returns to Scotland.