If the house was originally one house this might be considered exempted development.
However, if the house was for example a duplex (one above the other) from the outset then you may need permission.
Check with the planning officer to get the local authority's view on the matter.
You should also check the status of the house under planning to ensure that the work was compliant and be careful about buying property that may have been converted in a non-compliant manner.
Only buy if certs are offered and/or your team of professional advisors (archtiectural, engineering, building and damp specialist) have given you a preliminary costing for the work that suits your finances.
BTW, this house isn't in Templeogue is it - period, but unlisted?
ONQ.
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.