This branch of the Clan Donald had historically claimed ownership of land both in the western Scottish islands and, following the 1399 marriage of Irish heiress Margery Byset into the family, in County Antrim, north-eastern Ireland.

[5] Mac Colla's career would, despite the larger context of the Scottish and Irish wars, become defined by an effort to counter Campbell expansionism, and particularly to recover Islay and other lost MacDonald possessions.

[6] Mac Colla's military career was prompted by the onset of the long and interlinked series of conflicts known as the Wars of the Three Kingdoms, in which several heads of the Scottish and Irish branches of Clan Donald gave support to the Royalists and to Confederate Ireland.

[7] He initially took a neutral position in the Irish rebellion; he raised a mixed Protestant and Catholic force to protect settlers against the rebels, engaging his relative Mac Colla to serve as an officer.

[9] Present at several actions in eastern Ulster including the Siege of Coleraine, Mac Colla was wounded at Glenmaquin in June 1642: later that year he left the rebels and sought terms with the Scottish general Alexander Leslie, 1st Earl of Leven.

[10] Although he subsequently rejoined the Irish Confederates, Mac Colla appears not to have been given another military command until late 1643, when he returned to the Hebrides as part of an expeditionary force against the Scottish government, by this point in alliance with the English Parliament.

Argyll eventually dispatched a force of 600 under James Campbell of Ardkinglas to dislodge them, and Mac Colla's rebels were driven back to Ireland; a small garrison remaining on Rathlin Island was defeated in June 1644.

It initially fought its way through Argyll, raiding Campbell properties: by August, Mac Colla was finally able to link up with the King's Lieutenant, James Graham, 1st Marquess of Montrose.

The support of Montrose raised the standing of Mac Colla amongst the Highlanders, who looked down on him both as an island outsider and as a landless member of the gentry, rather than the ancient nobility they were accustomed to follow.

[18] While traditional historiography tended to emphasise Montrose's tactical genius, some more recent studies, notably the work of Prof. D. Stevenson, give Mac Colla a substantial share of credit for some of the victories.

Oral history and Gaelic-language poetry also gave Mac Colla a central role in events, and preserved stories such as his supposed beheading of the opposing commander Sir Duncan Campbell of Auchinbreck at Inverlochy.

An account of the campaign sent to Dublin, possibly written by Mac Colla himself or by one of his colonels James Macdonnell, stated that "throughout all Argyle, we left neither house nor hold unburned, nor corn nor cattle that belonged to the whole name of Campbell".

[20] For a time much of Scotland was in fear of his progress, with one contemporary observer writing: "There is nothing heard now up and down the kingdom but alarms and rumores, randevouses of clans [...] Montross and MacKoll in every manes mouth, nay the very children frightened".

The campaign petered out in a series of sieges of castles in Kintyre, and Mac Colla was eventually defeated at the Battle of Rhunahaorine Moss in May 1647, escaping with most of his troops to the Isle of Gigha and then to Islay.

Stevenson has suggested that Mac Colla first introduced the tactic from Ireland, refining it with the addition of a musket volley at a range of 25–55 metres, after which his men would advance obscured by the dense smoke from their own firearms.

[24] Past historiography often presented the charge as a direct descendant of an older Gaelic mode of warfare, which relied on shock attacks by an elite of heavily armed troops to break an enemy's line.

[28] It appears that not all observers were impressed with Mac Colla's military skill: the Scottish professional soldier Sir James Turner, another veteran of the Thirty Years War, judged him to be "nae soljer, tho stout enough",[26] and accused him of being "excessivelie besotted with brandie and aquavitae".

Mac Colla himself rejoined the Irish Confederates: he initially made plans to lead his veteran troops to Spain into the service of Philip IV,[30] though in the event nothing came of the proposal.

[34] After his death, Mac Colla became a figure of minor folklore in Gaelic Ireland and Scotland, with songs and melodies written in his honour in both countries, and many stories entering the oral tradition of the western Highlands and Hebrides, particularly in districts inhabited by the MacDonalds.

Mac Colla appears in And No Quarter, a 1937 novel by Irish author Maurice Walsh, which covers the Royalist campaign in Scotland of 1644–1645, told from the perspective of two members of O'Cahan's regiment.

Ian Lom in particular, as a Macdonald of Keppoch, was concerned to frame Mac Colla's victories as part of a specifically Gaelic military effort against the traditional enemies of Clan Donald, ignoring the wider Civil War context and the contribution of Montrose.