I would suggest you pay the fine or this collection notice.
Given the article in the Observer in the 2008 the Authority will be using a Debt Collecting agency to collect the tolls:
The AA in Ireland said the Republic's National Roads Authority will
use a Europe-wide debt recovery agency to seek out drivers from
the UK who fail to buy an electronic tag or to pay later via the
Conor Faughnan, AA Ireland's public affairs manager, [said]:
'If I take my car into central London even though it has an Irish
registration number the authorities there can track me down and have
used a European debt collection agency to do so.
'... The NRA is copying the London experiment and will fine UK
citizens who think they don't have to pay. Our advice on coming to
Ireland is that if you are going to be using the M50 buy the tag or
pay online quickly afterwards.'