Thanks Kingskerswell. This process is not easy and being so new to it, we are grateful for any insight from folks who have been at this for longer. We are leaning toward the theory that though they later became Presbyterians who were involved in the Magilligan Presbyterian Church when it was founded, the Caldwells from Tamlaghtard (or at least some of them), where with the Church of Ireland and rolled the 300 yards down the road to the Tamlaghtard Church of Ireland. Learning more about the politics of the Church of Ireland in Northern Ireland, and the political upheaval in the late 18th century in that region, there seems evidence to suggest that folks in the area could have switched churches during that period. As one of us speculated, perhaps the Church of Ireland became synonymous with the oppressive English rule and the baronet landowners and at some point in this period our own ancestors abandoned that church for the more radical Presbyterian Church.
This is one possible theory for why we have 3 Caldwells listed on the vestry of the Tamlaghtard Church of Ireland in the 18th century.