Whilst slightly outside your 3-5* requirement, don't rule out the Holiday Inn Express, which you'll likely pick up for under €100 in January. If you saw their summer pricing this year you'd consider it an absolute steal!
Its location is fantastic, on O'Connell Street and just a couple of minutes walk from the Post Office - a key building in the 1916 Easter Rising. Bus services to and from the airport go past the door. It's a relarively recent addition to the city, so although officially it'd be 2* the rooms are clean and in good condition, and the showers work well, so you're getting most of the things you'd expect in a 3*. You're not going to get a full Irish breakfast, of course, but if you need that put the money saved into buying it elsewhere.