The Land Conveyancing Law Reform Act 2009 appears relevant to your query:
http://www.irishstatutebook.ie/2009/en/act/pub/0027/print.html
Chapter three deals with party structures.
However, it may also be covered by the planning acts if the structure is connected to both houses.
In other words, the law may let them do some things but the planning laws may require them to get permission.
If the access is shared you may have common rights over the entire structure.
You want to be reasonable, but you don't want your place to look like a dog's dinner either.
You don't want to lose access to land or allow them to make an unjustified land grab, if that is what's occurring.
This is one of those queries which may touch on both the law of the land and planning law.
I think this needs a professional to look at it in RL, assess the various issues arising, and offer advice.
HTH
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.