Here is an example of one councils criteria http://www.westmeathcoco.ie/en/ourservices/housing/housingloans/
The tax paid on rental profits by landlord does not reduce his/her eventual CGT liability. They are two completely separate taxes and do not impact one and other.
Have you not tried your County Council for a mortgage, my cousin got one a few months ago from them as the banks all said they did not have a long enough savings record even though they had proof of rent and the rent payments way exceeded the mortgage payments. it was for the same amount that you are looking for with a lower SVR than the banks.