Note: With anything but the bigger hotel chains (eg Jurys/Comfort Inn), or places with a single, standard rate across the board (eg Bewleys), it's often worthwhile asking if "that's really your best rate", when booking - it doesn't cost much to try it, and many places will give a discount to anyone cheeky enough to ask 
The cheapest hotel within walking distance of the conference is probably Bewleys Hotel Ballsbridge, at 99EUR per room, per night. The Schoolhouse Hotel is somewhat more expensive, but has a very nice restaurant & pub attached.
Slightly further away, still on the hotels, the Comfort Inn on Parnell Sq is about a 40 minute walk, or ten minute bus journey from the conference hotel (46A bus route goes along Parnell Sq/O'Connell St, and stops directly outside the conference hotel).
For cheaper, non-hotel options, there are quite a few good B&Bs in the immediate area - the Bed & Breakfast Network has listings of some of them.
Pembroke Hall Guesthouse is pretty close by, and comes in at 68 Euro per night.
Cheaper can be gotten of course, but mainly in the City Centre, which is about a 20 minute walk away. One of the closest real budget hostels is Avalon House, and it's about a 15/20 minute walk away.
If you're searching online, try to keep to the "Dublin 2" and "Dublin 4" postcodes, and you can't be too far away. Some of Dublin 6 is very close by too. Temple Bar is a serious party-zone, so be careful - don't stay there if you're not into being out all night! Gardiner St and Talbot St have some very cheap hostel accommodation too - but you do get what you pay for, and they're not the best part of Dublin, to put it mildly.
If you want to make sure that any particular place is close by, or on an easy bus/rail route, feel free to get in touch with NoirinPlunkett, or colm@a.o