Scotch church was often used in Ireland to describe secession Presbyterian congregations. Presbyterianism was established in Scotland and brought to Ireland by Scots settlers in the 1600s. The links with Scotland always remained strong. All Presbyterian Ministers in Ireland were educated there until the Presbyterian Theological College was established in Belfast around 1845. Many of the Ministers were themselves Scots. There were several schisms in the Presbyterian church but one led to the creation of Unitarian/Non Subscribing Presbyterians and another to the United Free Presbyterian church. They often got their Ministers from Scotland (often from the Free Church of Scotland) and were known as Scotch churches, presumably to distinguish them from the mainstream Presbyterian church.
The former United Free Presbyterian Church in Cullybackey is still known to older folk in the town as the Scotch Kirk.
Secession congregations were not great at keeping records, and it’s common to find there aren’t any that have survived. No easy way round that. There's a graveyard attached to the former Non Subscribing Presbyterian Church in Antrim. I am due to look in it for somebody else. I'll keep an eye out for any Mackey graves there.