The Merrion's main entrance features top-hatted doormen and feels rather imposing and off-putting to normal visitors; but if you walk past all of this; you will find a staircase down to a basement door at the end of the block. This brings you in to the Cellar Bar which, while still rather posh, is nowhere near as severe as up at the front.
I've now been to rather a lot of hotel bars - I'd guess at 100 - and generally know what to expect. I've even been to another hotel basement bar with vaulted ceilings; but the layout here is quite different even to that - there's multiple separate vaulted seating spaces, some quite cosy and almost like a snug in a conventional pub.
Another surprise was the tap list - there are 11 taps here; with 8 of them being Irish independent brewery product - Rascals, Sullivans, O Brother and two hotel branded beers of unknown source (Untapped suggests Sullivans supplied also). Looking at past check-ins on Untappd has shown some slight changes in what was on draught here, but sticking with Irish breweries to the fore
Another five star hotel had surprised me by calling out Rye River on their menus as one of their premium Irish food and drink suppliers; so my guess here is that the Merrion sees offering Irish beer (and whiskey - they had plenty of Irish on display too) as part of a similar offering. This is something more venues should do.
The bar isn't particularly dear, but you may need to arrange with the waiting staff for a table even at relatively quiet times - this is also one of the dining options for the hotel. Bar seating is available and there are even complimentary bar snacks. Certainly worth a look, and interesting as it is effectively a craft beer bar, but completely unlike every other one in Dublin.