I believe that this is the kind of thing that the Land Commission Law Reform Act 2009 was written to assist in resolving.
Here is the link to the Act - read Chapter three and I hope it of some help.
http://www.oireachtas.ie/documents/bills28/acts/2009/a2709.pdf
I suspect that it may not rest on whether you can get access, but what this may cost in time and money to resolve.
If taking legal action, it may be useful to cause this to get into the Circuit Court as opposed to the District Court.
I have found understanding of the law and legal precedent a little hit and miss in the District Court.
Otherwise ADR - alternative dispute resolution - might be a preferred way forward.
In relation to the drainage, you should first establish if your drainage originally discharged onto their site - do you know the facts or are you repeating what your neighbour has alleged.
I don't mean to sound accusatory, its is important when considering anything to establish facts as opposed to working on assumptions or what someone else said.
The point being that this outfall appears to have been in place for some time and there is therefore some reason why it has come "live" now.
Several questions seem to arise; -
- If the outfall has been varied on their side, was this done by your builder with their permission at the time and can this be confirmed?
- If they are saying, no it wasn't, why wasn't it raised or acted on then - if they are saying yes it was, what is the current issue?
- IOW, has the outfall been done almost long enough to establish an easement - can this be done?
- Is the imminent establishment of a right the reason for the current concern?
Finally, I'd try to find out where this concern arose, themselves or a third party. You might be surprised at how far sighted some potential beneficiaries/family members can be when they are considering their inheritence.
HTH
ONQ.