I would hope that the advice on here would always be to disclose this to Revenue!
First of all, you need to get an exact picture of what you owe. Now, this might not be as bad as it first seems as there are a large number of expenses which you can deduct against rental income (have a good look around this website) For example any mortgage interest on the property can be deducted from the rent received. In many circumstances, this can make a large dent in the taxable amount.
After youÂ’ve calculated how much tax you believe is due, penalties and interest are likely to be added to tax owed. The Revenue can use their discretion to mitigate penalties. Voluntary disclosure is one factor which will count in your favour.
Depending on the amounts at stake, you may wish to consider bending the ear of an adviser (you can find a list on here I believe) who has experience in negotiating settlements.
Hope this helps (and good luck)