The Merrion is worth a premium over the FS-Dublin.

The Merrion has an absolutely top in-city location, yet still offers gardens and some outdoor space. I've stayed here half a dozen times over the past decade and have yet to have a service issue. The location is walk-to for just about everything in Dublin city-centre.

Restaurant Guilbaud has 2 stars and is, IMHO, the best luxe dining in Ireland. The hotel also has a more casual restaurant, and a charming lobby bar.

The gym won't knock your socks off, but it has been updated in the last 5 or so years, iirc. The indoor pool is nice.

The hotel is in FHR as well. Assuming you have the Platinum card, you'll get guaranteed late check out, breakfast for two daily, upgrade upon arrival if available (I've found them to be generous is in this regard) and the varying fourth amenity, which I don't recall for this hotel - but likely a spa or dining credit.

I've stayed at the FS and one other hotel in Dublin (the name of which escapes me), but this property, is without doubt, my default residence in Dublin.