My wife is descended from James DUNN, Sr., and Agnes, but Agnes's maiden name may not have been AGNEW. You may wish to review the data presented at http://genforum.genealogy.com/martin/messages/21809.htmlhttp://genforum.genealogy.com/martin/messages/21809.html, where fellow DUNN descendant Martin Graves presents evidence from the Benjamin Jasper MARTIN family Bible that indicates that the Agnes who m James DUNN, Sr., was the dau of James CROZIER.
The 1778 will of James CROZIER, discussed at http://genforum.genealogy.com/cgi-bin/pageload.cgi?pressly::crozier::622.htmllhttp://genforum.genealogy.com/cgi-bin/pageload.cgi?pressly::crozier::622.htmll, is problematic in that he names his dau "Agness Crozier," but Agnes had been m to James DUNN, Sr., since abt 1759, bearing her first child abt 1760. The normal way of naming a married female heir in Colonial days was to cite the married female's last name as the surname of her husband, not that of her father.
James DUNN immigrated to SC from Ulster, so if his wife was an AGNEW, she would almost certainly have been a descendent of your ancestor Samuel AGNEW who removed form PA to SC aft his father James's death in 1770. Interestingly, James DUNN and Agnes are both buried in Greenville Presb. Ch. Cem., Shoals Junction, Greenwood Co., SC, as are also, it is said, your ancestors Samuel AGNEW and his wife Elizabeth SEAWRIGHT.