As in any big and attractive european city accomodation in Barcelona is difficult and can be expensive...
Finding the right accomodation in Barcelona can be hard as renting is not as common as in other european countries. Most people rather buy(or still live with their parents) than to rent as on the long run this is more profitable. If you do not intent to stay longer than 5 years, renting will probably be your case. Look in the newspaper La Vanguardia on sundays www.vanguardia.es or do as the locals do, and walk around in your favourite area and watch out for postings on walls or houses saying "piso en alquiler" or "habitacion en alquiler" (flat to rent / room to rent) or "piso en veta" (flat to sell) and try to chat with people. Check also local supermarkets, irish pubs and posting in universities. Habitatge Joven www.habitatgejove.com a service from the city basically aimed at young people and students offers flats(you will have to go there physically). They have also a very good database on the internet where you can search an find a room in a house or flat to share. Oh-Barcelona www.oh-barcelona.com is another service that provides short term flats and/or rooms amongst other services. Another good place is the big notice board with a lot of posting in the CIAJ(Centre d'Informació i Assessorament per a Joves) at calle Ferran 32 (Metro Liceu). There is also a small-ad magazine called annuntis www.anuntis.com which has a aquiler (to rent) and a compartir (to share) section. Ads can also be found and inserted for free in Barcelona Metropolitain www.barcelonametropolitan.com a free monthly magazine aimed at english-speaking Barcelona residents which can be picked up in irish pubs, language shools, bars, consulates etc. Check also www.coleadministradors.com (they have a daily updated listing about flats) or www.servihabitat.com. Ambassadies and consulates and/or cultural institutions such as the german Goethe Institut or the french Institut Français might also have noticeboards and or housing services. Check also english bookshops as they often have a noticeboard. Finnaly you can of course also go to an agencia immobiliario (real estate agencies) but you will have to pay a commision which can be up to 2 months of the rent.
WARNING - RIP OFF! A lot of perfectly sounding ads you read in the papers(often marked with an *) will lead to "agencies" which will tell you that they have exactly what you are looking for and ask you to come by. Most of the time these "agencies" will try to sell you a list of appearingly available flats/houses that perfectly match your search criterias. These lists are often a simple copy and paste version of old (at some stage available) offers from anywhere. Don't loose your time and money here!