I'm not sure I'd ever expect a 750 or 1000 ml bottle to be sold on a licensed premises without a significant mark-up compared to an off-licence.
Likewise with the Lagunitas. You're getting it from a keg in the hopefully enjoyable surroundings of a busy pub. Yes, you could sit at home with two bottles of it from the off licence instead (but why stop there - Aldi or Lidl beer is even cheaper).
When is the mark-up too much?
The market will answer that, I suppose.
Something tells me that, as with the other Galway Bay pubs in Dublin, their pricing is on target more or less. Plenty of people out there with money in their pocket for this kind of beer.