Hi moryan
Irish research can be very difficult. If you have no clues as to the county, if it's a common name etc it can be nigh on impossible without anything more to go on.
Was it a couple with children who came over to Scotland? Or just a single person who married in Scotland?
Have you found all possible Census - and none of them say any more than "Ireland"?
If he/they died in Scotland from 1855 +, what does the death cert say about the parents?
Cheers
AMBLY