Steve Barber makes some 28mm Gallowglas link
So does Timeline Miniatures link
As does Royal Lion Miniatures royallionminiatures.blogspot.com
You might be able to use some of the Islemen/Highlanders from Elite Miniatures link
Redoubt Miniatures has both Gallowglas and Kerns link
Warrior Miniatures has a range of large 25mm Border Reivers link
Essex has an Irish Kern link
Irregular has some 28mm Scottish Highlanders, include a musketeer link
You might want to check companies that have either a War of the Roses line, an ECW, or even a 1745 line. You can often find Irish or other suitable figures in those lines too.
Happy hunting.