David, Knogh is short for Knogher. It was an old Irish name meaning Connor. Some of the O'Connor family were known as O'Knogher. Knogher is pronounced "nogger". It was fairly common in the Hearth Money Rolls for 1663 and 1665.
A Hugh McCaffrey on the 10th of July 1741 married a Mary Dehart in the 1st Presbyterian church,Pa. He could be the same Hugh who is found in Loudoun Co. Va. Best regards Oliver