If you want to become compliant, you will have to make a return for every year the property was rented, to just address a few years would be very foolish. If Revenue asked questions, you could be looked at as someone who had made a false declaration, rather than someone who was late with their returns. If you go to confession better make a full confession.
If you were making substantial capital repayments on the mortgage, you could have been making large profits even though the cashflow from the property was negative. On the other hand rents 10 years ago were low, so perhaps the profit wasn't large. It is impossible to say without much more info.
The NPPR is possibly your biggest issue. The penalties were draconian. However AFAIK there is a time limit on these. That us the first issue you need to understand before making any declaration.