I'm afraid that the information you're giving isn't enough to answer you fully or properly.
Building a garage within the curtilege of a property may be exempted development.
Raising the roof of an existing garage or making the walls gabled higher and putting windows in them isn't specifically listed as exempted development.
You don't say what height you intend the roof of the garage to be, or whether the garage had permision, what size is it, whether it is exempted and, if so, what other development, exempted or otherwise, may have occurred.
All these things are material to whether it needs permission or not.
The 2001 planning and development regulations say the following:
From:
http://www.irishstatutebook.ie/2001/en/si/0600.html
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CLASS 3
The construction, erection or placing within the curtilage of a house of any tent, awning, shade or other object, greenhouse, garage, store, shed or other similar structure.
Conditions:
1. No such structure shall be constructed, erected or placed forward of the front wall of a house.
2. The total area of such structures constructed, erected or placed within the curtilage of a house shall not, taken together with any other such structures previously constructed, erected or placed within the said curtilage, exceed 25 square metres.
3. The construction, erection or placing within the curtilage of a house of any such structure shall not reduce the amount of private open space reserved exclusively for the use of the occupants of the house to the rear or to the side of the house to less than 25 square metres.
4. The external finishes of any garage or other structure constructed, erected or placed to the side of a house, and the roof covering where any such structure has a tiled or slated roof, shall conform with those of the house.
5. The height of any such structure shall not exceed, in the case of a building with a tiled or slated pitched roof, 4 metres or, in any other case, 3 metres.
6. The structure shall not be used for human habitation or for the keeping of pigs, poultry, pigeons, ponies or horses, or for any other purpose other than a purpose incidental to the enjoyment of the house as such.
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These regulations may be amended by later ones, so its best to use them only as a starting point.
I advise you to talk to the planner, or appoint an architect to draw up plans and talk to him.
I think you may need permission, but its the planner's call.
ONQ
[broken link removed]
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.