I've had a look in a dictionary and Cuillin is already plural. So the Cuillin, not the Cuillins, I guess.

Cuachag nan craobh is a gorgeous sad song probably written by William Ross, pining for his beloved, Marion Ross, who'd married another. The story goes that William died of a broken heart one night and at that same moment, Marion Ross was woken up by a knocking on the door. Taking her candle, she went to open it and the draft caused the candle flame to ignite her nightgown and she died the same night as William. Trust the Gaels to have some nice dramatic stuff!