The Mercer Hotel at Royal College of Surgeons Ireland is a moderately priced but very nice hotel one block away from St. Stephen's Green and Grafton Street. The hotel is owned by RCSI (and thus offers discounts for those who are connected to the school) but is open to everyone. It is a full service hotel, and should NOT be confused with Mercer Court, the RCSI housing facility, which is rented to travelers for cheap over the summer.
Our room was very clean and very big for a European hotel room. The bathroom may have been the best part—it was very spacious and had a very good sized tub-shower. Our room included a Double bed, a desk, a phone, a small TV (with about eight channels), a coffee maker, a fridge, and a bathroom en suite. The bed was hard but comfortable, though I was not too thrilled with the hard pillows.
There is a restaurant downstairs, and the food is very good. Breakfast is complimentary with most room rates. The staff at the front desk is very nice, and they are more than willing to store your bags before or after check-in or check-out.
The great part about the Mercer is, by far, the central locations. Yes, the hotel is very nice (though by no means luxury) and certainly a fine place to stay, but the key is the proximity not only to Grafton and St. Stephen's Green, but also Temple Bar, Trinity, Christ Church, Dublin Castle...you get the point. There are plenty of great (and not necessarily touristy) restaurants and pubs within easy walking distance. It simply does not get more central than the Mercer.