anniem,
I'm not sure this is the right forum, as I think this is a legal issue.
If there was some way for you to quantify your lost business [assuming this is the case] it might be possible that the courts would grant you an order restraining him from interfering with your customers.
I think the attitude of the Gardaí is poor in this instance as a legitimate business is being interfered with.
I wonder could your local chamber of commerce help.
Failing that there may be irregularities in the way your "enemy" is running his business.
Vexatious creatures often seem to cut corners to increase their profits.
If you discovered any breaches of regulations of whatever you could highlight them to the proper authorities.
You may be able to get him shut down.
I know this sounds extreme, but its down to how badly you want to trade from there and whether you can deal with any adverse fallout from this course of action.
Failing this, the location doesn't sound too good, with restricted parking and an abusive neighbour - could you find alternative accommodation?
FWIW
ONQ.