Hi Brian,
We are probably distantly related if your David is also from Cookstown. There was a David Acheson (farmer) resident in Cookstown around 1730 when the town offically formed. He undoubtedly had descendants and relatives in the area, some of whom were named David. The Derryloran parish register is online http://www.derryloran.com/ but I did not see your David and Matilda, or any Susan's in it. A y-chromosome DNA match between myself and one of your male Acheson relatives would confirm the link if a paper trail can't be found. My own DNA is already on y-search. My 2nd great-grandfather David William Acheson and his wife Mary Neill also had children born in the 1860's, most of whom immigrated to New York City/State and New Jersey and fanned out from there. The names James, Robert, William, David, Thomas and John are all common in our branch of Achesons. My email is acheson@gwi.net if you prefer to writer me more directly.

Richard