Everywhere mentioned are places people attending Nimhneach have stayed in. We live in town ourselves so have no direct experience. Some folk have good luck with the deal sites as well
The Maldron in Smithfield is a straight walk down the Quays, or the Luas tracks, 10/15 mins or so.
The Aisling Hotel near Heuston Station seems to have availability when other hotels don’t.
Serviced Apartments can be a good option if you are 2 or more. Regulars have been known to hold parties in these.
Nearby hostels include:
Sky Backpackers off Bachelors’s Walk
Abbey Court Hostel
About 15 minutes away, the Lower Gardiner Street area has several hostels.
These include Globetrotters