It will all come down to the valuers report - he/she will recommend one of the following;
1. The property is mortgageable but you must undertake to carry out XYZ works within (say) 6 months
2. The property is mortgageable but the lender is advised to retain €XXX until the following work is completed - this will involve a second inspection after which the retained funds will be released.
3. The property is unmortgageable.
In the scenario you've outlined I would guess at no. 2. If you are extending the property you should show the valuer the plans and estimates as he/she will then be able to estimate a "when finished" value and the lender will be able to use this to determine the final mortgage amount you will qualify for (subject to income and other criteria) which is easier on all parties than having to keep going back for additional funds.
Sarah
www.rea.ie