Logainm.ie, the database of Irish placenames, has a map and translation for Slieve Gullion - it translates as Mountain of Holly Trees or of the Holly Tree (I'm a bit hazy about genitive singular and plural, sorry).
I might mention that the Great Wood of Leinster, otherwise known as Cullenswood, was heavy on the holly. (Leinster is of course south of Armagh by a good few kilometres.)