I looked through 1841 census for John and for Thomas with no luck. I expect you have seen 1851 census:
Clenagh, Lezayre district 3:
John Cretney, head, 67, farm labourer, b. Marown,
Ann, 70, wife, b. Ballaugh,
John J. Boyd, 9, nephew, b. Lezayre.
Janet.