If they were Catholic, why would the ground have been consecrated by an Episcopalian bishop?
GR2 - You think they wouldn't have made it into any Church of Scotland register? As I said earlier, no parish register has survived for Crawford & Leadhills, so you can't be sure.
The rule was, anyone - no matter what denomination - who was living in the parish at the time of death, was allowed interment in that parish burial ground. Prior to the 1847 epidemic, at least a few of the Irishmen must have died there.