This is loosely covered in the Planning and Development Act.
Planning and Development Act (2000) - PART I - Preliminary and General - Exempted development 4.
"(h) development consisting of the carrying out of works for the maintenance, improvement or other alteration of any structure, being works which affect only the interior of the structure or which do not materially affect the external appearance of the structure so as to render the appearance inconsistent with the character of the structure or of neighbouring structures;"
Now I will say the above is completely open to interpretation, but I would be of the view the change you are suggesting would fall neatly into the above exemption. This is however not prescribed in any further detail in the Planning and Development Regulations, and any trigger happy planner in the Local Authority could take another view on what you are doing and look for a retention application or for you to restore the house to its original condition.