I suspect his parents were William Cowley + Mary Clague - if so then William's decree should help :- d 18 nov 1771 by fall off quay in douglas;7ch wm,thos,patk,john,isabel,jane + robt all ua uncles patk,thos + robt cowley;fa thos cowley (also prin creditor);wife mary;inv,sale + some dispute re fam selling goods
(found under 1772 Episcopal wills - think full decree should be online somewhere)
The uncles suggest William was born in 1730 and his mother was possibly Alice Brew but I never found a marriage for Thomas + Alice whose name was mentioned as mother of a child by a Thomas Cowley at the right time + in same parish as other siblings - however as not named as a supervisor in decree suggests she is dead