The earliest known ancestor was William Cahill, who was born near Dublin, Ireland, between 1710-1720. He immigrated to the U.S. sometime before 1735-36. He married Ellen O'Connor, who was born around 1714-15 in Maryland. They were of the Catholic religion. William Cahill died between 1782-1788. His will was probated in Montgomery Co. MD. Presumably, Ellen also died in MD.

The children of this marriage were as follows:

Philip E. Cahill, born 1736

Infant Cahill, born 1737

Richard Cahill, born 1738

Mary Cahill, born 1740

Dennis Cahill, born 1741

William Cahill, Jr., born 1743