I wouldn't expect all the Blairs in Ireland to be related. That wouldn’t be very likely. It’s a common Scottish name and I would expect many different and unconnected families moved to Ireland in the 1600s. (In the 1841 Scottish census, there are nearly 4000 people named Blair, scattered right across the country. That gives you an idea of how common and widespread the name is).
The majority of Scots who moved to Ireland settled in the counties of Ulster, and the majority were mainstream Presbyterian though there were some RC & Episcopalians Scots settlers too.
The Reformed Presbyterians (Covenanters) in Ireland don’t really have any records for the 1700s. The first ever RP Minister in Ireland was the Rev William Martin who was ordained in Ireland in 1757. His “presbytery” was all of Counties Down, Antrim, Londonderry & Donegal. He lived near Kellswater in Co Antrim but preached all over Ulster. Initially, he had no church and preached in barns or the open air. He kept no records in Ireland (or none that have survived anyway). The first RP church was his built at Kellswater around 1760. A second Minister, Matthew Lynn, was ordained in 1763 and took over the Bannside part of Rev Martin’s presbytery (ie Cos Derry, Tyrone & Donegal). By the time Rev Martin left for America in 1773 with 5 shiploads of emigrants, 4 more Ministers had been ordained. A grand total of 6.
However by 1779 the presbytery collapsed because 2 Ministers had emigrated to the USA and 2 others had died. It was re-established in 1792 with 6 Ministers. There were still only a handful of churches: Kellswater, Letterkenny, Conlig, Dervock & Kilraughts being ones that I know of. Around 1793 the Ministers for Kilraughts and Kellswater both had to go into exile for preaching treasonous sermons. Back to 4 Ministers, all covering huge geographic areas. Some itinerant Ministers from Scotland came to Ireland from time to time and conducted baptisms and marriages, but they kept no records of those events. (Or if they did, none has survived).
So the picture that emerges for the 1700s, is one of an embryonic organization, with few church buildings, a handful of Ministers (some of whom died or left at short notice), and even fewer records.
A fellow researcher told me that the Ballylagan RP congregation near Coleraine, Co Derry, may have some pre-1800 records. If so, they are the only 18th century RP records that I have ever heard of. And if they do exist, PRONI (the public record office, in Belfast) don’t have a copy, and so the only copy must presumably be held by the church.
Kellswater RP church has Kirk Session minutes starting in 1789 (but no baptisms or marriages). They are the oldest RP records I have seen.
In short, if your ancestors were Reformed Presbyterians who lived in Ireland in the 1700s and early 1800s, I don’t think there are any church records you can usefully access. Tracing your family maybe tricky. And without parents names there's no way of being sure whether any particular family is the correct family.
