It's been almost 10 years, but...
My g-g-g grandfather was also named James Slater Jackson, from Northern Ireland. He isn't the same James Slater Jackson because he emigrated to New York about 1843. But the name similarity is striking, so perhaps they were some sort of cousins ...
His father was also named James Jackson (I don't know the middle name), born around 1800. The Jacksons were Presbyterian. James Slater and his siblings were said to have been born in "Laurel Lodge" somewhere in County Tyrone. My James Slater referred to it as "the ancestral estate of the Jacksons in County Tyrone", so it would have produced numerous Jacksons going back into the 1700s. This structure seems to be gone now, and I don't know where it is (couldn't find it on Griffith's Valuation maps) but I suspect it was near the border with Armagh for several reasons that I won't go into now. Suffice it to say that James the father was married somewhere in Armagh in 1827. So if your James and mine are connected, and geographically close, I would look for him in churches near the border with Tyrone.
