I hadn't seen this before but according to here:

http://www.offshoreradiomuseum.co.uk/page378.html

That there WAS a test transmission in New Ross Harbour during the early hours of 17th December 1983.
However the frequency used is not stated.
It could have been in the small hours of the morning on 729 after RTE had closed down.. (or some other frequency??). If (I assume) just playing non-stop music it would not have drawn attention, unless perhaps a radio enthusiast living nearby just happened to be tuning around MW and due to its strength realized it must be a local signal (rather than nighttime skip of something from another country).