Hi Colly,
I'd recommend that you look into your tax situation as soon as possible. You're obliged to file a return on your rental income even if you don't make a profit. In my case, I'm a PAYE worker but I also file an annual return where I state the amount of rent received, the amount of mortgage interest and other expenses paid etc. The revenue then work out what (if anything) I owe. Sometimes they owe me money and sometimes I owe them but regardless, they will fine me if I file late.
Regarding making a loss, you need to be very sure that this is the case. Suppose your monthly rent is €800 and your monthly mortgage payment is €1,200. You might think that since you are taking in less than you are paying out, that you are making a loss. However, the €1,200 is made up of capital and interest payments (lets suppose 700 capital and 500 interest) and only the interest bit is deductable. Furthermore, not all of the interest is deductable, currently only 75% of it is. So lets imagine the following:
Monthly rent: 800
Annual rent: 9,600 (800 x 12)
Monthly interest: 500
Annual interest: 6,000 (500 x 12)
Deductable annual interest: 4,500 (75% of 6,000)
Your annual gross profit: 5,100 (9,600 - 4,500)
I apologise if this is stuff that you already know, I thought it was worthwhile laying it all out for the benefit of other readers of the site.
As regards your historical position, if it turns out that you don't owe anything, I'm sure revenue won't be too worried about not filing returns. If you do owe them, then the fact that you have voluntarily disclosed this will mean that they should be prepared to waive fines and possibly reach a compromise on what you have to repay.
Good luck,
Phil