What is the info leading you to the IOM?
In the Kirkmabreck 1861 census, there is a 7 year-old William Coull with his 62 year-old widowed grandmother Sarah Johnstone a hawker-pauper. He was born in Creetown, she in Ireland. In the 1871 census he is recorded as 17 year old unmarried William Cowell with his 72 year-old widowed grandmother Sarah Johnston. His place of birth is again given as Creetown, Kirkcudbright. He is a Granite Heaver.
In 1881 he is still in Kirkmabreck on Norris St. with his wife Isabella 23, and children James L 4, Jane S 3, Sarah J 5months and also gives his birthplace as Creetown.
Possibly his father came from the IOM.