I've seen several reference's to Irish Calvermen from this time period. I just picked one up here in the States and I REALLY like it! The caliver is much lighter than an Aquebus or a musket.
Mine was made by The Rifle Shop in Oklahoma as a kit. I purchased it already built and lightly used. They make the best I have ever seen. http://www.therifleshoppe.com/
Swifty is right, they are not cheap. I was luky to stumble into a used one.
"...The O'Brennans, a sept of thieves without any right or title, ... were a perpetual disturbance to the peace of the county,"