This is a bit of a puzzle-Stuart Reid has argued that they were a conventional pike/shot armed unit, but I've seen good argument that they were not.
The force was raised by Ranald MacDonnell, Marquis of Antrim.
Although Irish with experience in the Confederate armies, ,they were mostly tenants of Antrim. Displaced by the Scots in Ulster and unhappy with the cessation of the Confederate war, I'd say they were redshanks with a grudge.
Whatever their training or experience in the Confederate armies they do not seem to have brought the pike/shot tercio of O'Neill with them to Scotland.Antrim initially raised 1600 men, but could only send 800 due to lack of shipping-maybe the pikemen stayed home?
Once they were in Scotland they appear to have looked and acted the same as the highland MacDonalds (the reference of Spalding to Montrose "This Livetennand wes cled in cot and trewis as the Irishis wes cled. Ilk one had in his cap or bonet ane rip of oatis quhilk was his sign.")
The report of trailing pikes at a funeral was in Aberdeen. No regiment is named, and Montrose had kept the Irish out of the town as the population had threatened to flee if they came back!
Wishart says Montrose.."..placed MacDonald and the Irish in the centre. This was a very prudent disposition of his men, for, had he placed the Irish on the flanks, as they had neither pikes nor swords, they would otherwise have been much exposed to the enemy's horse."
So I'd say-Antrim's landless tenants, overwhelmingly Ulster scots, are released by the Confederates(no doubt they were a drain on resources and a bit of a handful)as mercenaries for Scotland.
Although possibly trained and armed as conventional troops, once in Scotland they join their cousins and prove far more useful as musket/sword/targe armed shock troops.