yep im in ireland at the moment and its fairly busy, tourist-wise. id book at least two days ahead for weekend nights for sure, esp in places like killarney -- i was there a few weeks ago and backpackers were being turned away as every hostel was full.
in dublin - i recommend jacob's inn -- right near the centre of town/temple bar areas, next to train station. basic rooms but they do have lockers for storing luggage, and are clean. you get breakfast (muffin n coffee/tea) as well. and if u book on the net (try hostelbookers.com, hostelworld.com or the one on the tpunk main site!) they are cheap as chips!
(oh and if u want to meet up and buy me a beer, im living in dublin at the moment so pm me..