There is a reference in the biography of Shellburn M. Kepner, husband of Emma (McCulloch) Kepner and grandaughter of Samuel McCulloch, that Samuel McCulloch was born in Northern Ireland.
"History of the Friendly Sons of St. Patrick and the Hibernian Society For The Relief Of Emigrants From Ireland: March 17, 1771 - March 17, 1892" (The Hibernian Society, Philadelphia 1892)(page 475 of the book and page 688 of the downloadable pdf file).
The importance of "The History of the Friendly Sons of St. Patrick" and the biography of James McCulloch which is included therein is that it gives us a reasonable explanation as to why James McCulloch does not show up in the 1830 Juniata County Census of 1830. This history also tells us that James McCulloch probably stayed behind in the Philadelphia, Pennsylvania port of entry when he and his brothers Samuel, William and John McCulloch arrived in America. Most importantly, the history tells us that the McCulloch brothers left their sister, Nancy (McCulloch) Gilmore and her husband, Joseph Gilmore, in Belfast, County Antrim, Ireland and therefore, Belfast, County Antrim, Northern Ireland is the most likely candidate as the ancestral home andpoint of departure for the McCulloch brothers.