By the 18th century, the Preston surname was scattered all over Ireland although they were very early associated with Meath. This may be your permanent brick wall, rarely is any research possible in Ireland pre-1800 unless it's within one of the few big cities whose churches kept good records. However Nick, the fact that you are a direct male descendant, I would recommend a YDNA test from FamilyTreeDNA (they specialize in the Y-line genetics, its not one of those "ethnicity test" fads). This will put you in touch with either close or far Preston connections as well as people related to your Preston line before Preston was even the surname. It's definitely possible to narrow down a geographic area of origin this way. If you take the test and need any help, dont hesitate to shoot me a message.