Hi Narky, unfortunately Revenue does not care how much you have to sub the mortgage! We were in a similar position - we moved abroad and rented out our family home (only property that we own, we are renting abroad).
Despite subbing the rental income at one point by almost a grand a month to pay the mortgage, we still would have been liable for tax on the 'profit' we made by renting out our home but, as our mortage was relatively new and a giant part of the repayments were interest and as we did about 5 grand worth of repairs to the house the same month as the tenants moved in which we were able to claim against over a 4 year period, we ended up not owing anything.
The sum you need to focus on is the % of your repayments that are going in paying off the interest on your mortgage rather than the differential between your mortgage repayments overall and your incoming rent. Your yearly statement from the bank should tell you what proportion of your repayments last year were interest and what proportion was capital. If your mortgage was taken out recently, the chances are that you are paying a significant proportion in interest and therefore will be able to offset a significant proportion of your rental income.