After plundering Perth the little army of Montrose crossed the River Dee at Crathes, about 15 miles north of Aberdeen and marched on the city. On September 13, 1644, they found about 3000 foot soldiers, 500 cavalry and 3 cannon facing them.

Here, too, the Irishmen seemed to be at the heart of things with Captain John Mortimer adopting another unusual stratagem of mixing foot soldiers within the cavalry and adopting the wild charge upon the Covenanters.

Captain Mortimer would later follow Montrose into exile and return in 1650 to fight at Philiphaugh. Elsewhere, Sir William Forbes of Craigievar charged his cavalry at Alexander McColl's troops who fell apart and let them through, then turned in on them and slaughtered the rest. The Covenanters lost about 800 killed.

After Aberdeen, the victors returned to Blair Athol where a detachment was sent to the assistance of the Irishmen left at Mingarrie and Lochaline who were under attack from Argyll.

On the face of it Argyll was seeking to save the three Covenanter ministers who had been captured and which would have been readily exchanged for Alexander McColl's father and two brothers whom Argyll held captive. He refused to exchange and pursued his attack as the castles were critical to the defence of his lands. But when the relieving Irish troops came near, the assault was abandoned.

Arising from this failed siege was the joining of the Clanranald of Lochaber, including all the MacDonnells of Garragach and Keppoch, and the MacDonnells of Glencoe, to the Royalist cause.

Fivy, October 28, 1644

Meanwhile Montrose's campaign restarted by marching from Blair Athol towards the lowlands when the armies cam across one another at Fivy, west of Montrose, on October 28, 1644.

Argyll and his troops had been tracking the movement of Montrose's forces and a respectful distance and had occupied himself by devastating the districts of Lude, Speirglass, Fascally, Don-a Vourd and Ballyheukane, firing the land to Angus and northwards to Dunnottar.

His force of 1000 claymores, 1500 militiamen and seven troops of cavalry under the Earl of Lothian came upon Montrose and his troops. Once again it was the Irishmen, this time under Captain Manus O'Cahan who dislodged Argyll's troops from an important firing position. The Covenanters again turned and fled with many of the cavalry especially, left dead.