Hi Tony,
Because of the number of Cork(h)ills who were in the IOM at that time it is important to be certain you have all the information for each generation before going back to the one before. As you are certain that Joseph, son of Thomas, is your ancestor, do you have a copy of his birth certificate? If so, you will have his mother's full name and address? From this you can order a copy of Thomas's (English?) marriage cert., which will give his father's name and occupation. You need more than census records to be sure you have the right branch.
Don't worry about varying ages in the census records - many people reduced their ages as they got older. You were very lucky having such a precise age for Thomas in 1841. It is unlikely to have been a late baptism without it being noted on the parish register.
If you have Joseph's mother's full maiden name from his birth cert., and Thomas's marriage details, would you please post these?
Sue