There's been a series of "lasts" as I come towards the end of the list - last pub or hotel in this chain or that mostly. This instead is the last of a type - this bar operates under a Railway Refreshment Rooms licence.
In theory, these are only meant to serve customers with tickets for a 10 mile distance; but this is effectively neither checked nor enforced; and anyway - any Leap card can become a ticket for this when tagged on (However, when Good Friday drinking was restricted, any Railway Refreshment Rooms that were actually open did check for tickets)
This was, I believe, the final 'refreshment room' actually operated by Irish Rail, under their Network Catering division as Brasserie na Mara; but when Irish Rail ceased doing it's own catering, the restaurant closed with it.
Hartleys opened here in ~2008 and has traded since, with drinkers accommodated on a terrace and diners inside in the Victorian railway dining room. The drinks are exceptionally expensive, even for Dun Laoghaire; but there is often live music and the terrace is an interesting place to sit and view the harbour, so there may still be a reason to go here for some.