The Order on Battle lists and additional notes can be found on the Inverlochy (1645) OoB page.
The Covenanter Deployment
The center of the Covenanter’s line was placed on some slight rising ground to the south of Inverlochy Castle. Looking at topographic maps this appears to be a small rise near the modern rail station labeled as “Torn Na Faire”. It would be fun to translate this as Fairy Tower but I believe it means (in Scots Gaelic) “Hill of the Watch”. I leave it to others to correctly translate it.
At the very crest of the hill is Argyll’s musket and pike regiment, probably the best Covenanter troops present. One or two guns are mentioned and these are placed in the gaps between the center and flanks, a common deployment of the time. These were likely the very light frame guns often used by the Covenanters:
Behind Argyll’s is the mass of the Clan Campbell levy acting as the reserve. Little is known about their organization (probably in extended family groups). I have chosen to place them in three equal groups.
The two wings mirror each other. The elements of the two lowland regiments (assumed to be conventional musket and pike units) anchor each flank. The Covenanters, having no horse, apparently had some concern about Royalist horse and this would be a good position for pikes. Interior to both lowland regiments I conjecture a Campbell highland warband (more capable than the highland levies in the center).
Forty or Fifty Musketeers were detached to form a forlorn hope in the castle itself. The castle had gone through a series of destructions and repairs (the last perhaps seventy yeas before) and, while providing a strong position, was far from unassailable.
The Royalist Deployment
Montrose’s army had arrived during the night and rested on the eastern slopes of the Ben Nevis massif. Likely they were placed in battle order awaiting the sunrise.
MacColla took command of the right wing with Laghtnan’s Irish regiment (which I show as the only Royalist unit with both pikes and muskets). While not mentioned in contemporary references a group that would later be styled “MacColla’s Life Guard” is also present (the size of Laghtnan’s is reduced to allow for this).
The Irish musketeers of O’Cahan’s foot, supported by a warband of Maclean highlanders is placed on the left wing. Since in the contempary accounts O’Cahan made initial contact, I show them slightly advanced of the rest of the Royalist line.
Montrose is present with the center which is drawn up in two lines. Inchbrackie’s Atholl highlanders and MacDonnell’s Irish comprise the rear line with the mass of Clan MacDonald highlanders (and allies) shown in the front line. With the exception of the Irish, who were reported deployed in the rear line, the organization of the rest of Montrose’s center is conjectured.
The horse of Sir Thomas Ogilve, son of Lord Arlie, are also in reserve, but I have placed them well on the right in preparation for the sweeping move they made to cut off Inverlochy Castle from the Covenanter army.