I think you should have started this thread with the above information.
The information on the house boundary sounds dodgy to me.
What sort of conveyancing leaves you in this position?
Either you were sold a pup or didn't check it out fully.
You need to take competent professional advice.
You may need an architect to attend and swear.
OTOH if you inherited this property you owed it to yourself to investigate the lease and any maps associated with the transfer.
Assumptions based on observation may be irrelevant where there is a legal document to peruse.
The condition of the hedge may irrelevant in a boundary matter.
Now, all that having been said, there is a new law in the land called -
The Land and Conveyancing Law Reform Act 2009
Read Chapter 3 Party structures and you may wish to instruct a solicitor to prepare your defence partly based on the provisions of this Act.
I am not au fait with it all yet, but boundaries seem to have entered a whole new ball park with this act and you may find the rules have changed to help soften your neighbour's cough.
Depending on the condition and importance of the boundary two conflicting piece of legislation may also arise, both stemming from the Planning realm.
Protection of historic hedgerows and boundaries is a Green agenda which in South Dublin Co. Co. for example is enshrined in the development plan.
However there could also be an argument in relation to derelict sites such that the hedge provided harbourage for rats and vermin or in general was contributing to the delinquency of an area [I made that phrase up, but it sounds the part].
Again, you should instruct your solicitor to review all such possibly helpful matters and advise you on your defence.
ONQ.