The mortgage you will get will be based on the income you have even if you are self employed. If you want a pension mortgage then, the mortgage company will usually ask for the pension fund in projected terms be at least twice the mortgage amount on retirement. It does not have to be equivalent to 25% of the fund value on retirement. Under pension rules you can take more than this out however it will be taxed. You also need to have a guaranteed life long income of €12,700 per year ( from state and or other pensions) or invest €63,500 of the fund into a AMRF untill age 75. The rest of the money in fund is yours.
The above examples are for Personal Pension Plan. The rules are different again of you have a Company Pension Plan.
Hope this help!
Anto