My McIlvaine line I can trace back to County Antrim, Ireland and Ayrshire, Scotland. Proof gets a bit murky in Scotland, though; I have no proof of relationship to the McIlvain who was "Laird of Grimmet."
The last of my direct McIlvaine line was my ggg-grandmother, Mary Caroline McIlvaine, b. 1820 Harrison Co, KY, who married Ranson Perry Barr in Decatur Co, IN in 1843. Mary's parents were Samuel Hodge McIlvaine, b. 8 Aug 1790 at Elkhorn Creek, Woodford Co, KY, and Elizabeth Nesbett, b. 1780 at Indian Creek, KY.
Samuel's parents were John McIlvaine, b. 1742 PA, d. 1813 in Harrison Co, KY, and Elizabeth Hodge, b. VA. Samuel had brothers William and James J. William was the father of Moses H. McIlvain; I have no research on James. John McIlvain was the son of Moses McIlvaine, b. 1723 SC, d. 1761 Lancaster Co, PA, and Agnes Miller, b. 1723 PA. They married in PA in 1740.
Perhaps you can link to this line in PA or KY. I'm not sure how far apart Bath, Harrison and Woodford counties are.