After a few days of looking in this newspaper, I could notice that the same ads would be there in for days at a time. Another good source is on the web site, Dublin Accommodation Finder, The USIT shop near O'Connell Bridge also has an accommodation notice board. It might be an idea to look on the accommodation notice boards in Trinity, and UCD in Bellfield if you can. Keep an eye out in shop windows and possibly even church newsletters. Competition for shared accommodation is massive.
Rooms are also very expensive.
I was living in a single room in Milltown that was costing me 300 per month. I now live in a room in Portobello. It is twice the size of the one in Milltown is only 20 more a month. The prices for rooms range form about 250 to 350, depending on the area, size of the room etc. Generally one month deposit is needed, sometimes more, so have at least 800 put aside to start up your accommodation.