As a Diamond you will get free Breakfast at all properties and you may receive a room upgrade; usually just a higher floor or slightly larger room. In terms of view from the room don't expect too much, probably the best view is from the high floor river-view rooms at the Hilton Garden inn.
Note there is not a single property in Dublin with an Executive lounge.

My opinion in short:
Conrad Dublin: Good Hotel, it is a smaller Conrad compared to the average but rooms are good, nice breakfast and service. Great location short walk to city center main shopping area.
The Morrison Dublin, Curio collection. Great location in a shopping and restaurant/bar area, good breakfast and service, rooms are ok, some with minimalist design.
Hilton Garden Inn Dublin: Overlooking the River Liffey, a bit further away from the main shopping area but still walkable along the river or by crossing the pedestrian bridge. Breakfast is basic.
Hampton by Hilton: Slightly further away from city center but still in walkable range and in front of a tram stop. Breakfast is better than average for a Hampton but rooms are very very small.
Hilton Dublin: Overlooking a canal and just besides a tram stop. Not too far from the city center, you can reach the main shopping area in 15/20 minutes walk. Breakfast is ok,

I would not suggest Hilton Dublin Kilmainham And Hilton Dublin Airport for touristy stuff.