It would be very unusual for an undocumented migrant to volunteer their details on a census form. According to the Migrant Rights Centre of Ireland there are 20-26,000 people in the country with no documentation. Why would they want to draw attention to themselves?There 'could' be, there may not be too!! 'Illegals' if you wish to use the word at least spell it correctly.
And, given that the Central Statistics Office only issued 8 summonses over refusal to even complete a 2016 census form, it would appear that there is very little of consequence in not completing a form correctly.