Any suggestions for a b+b or guesthouse in Dublin for £30 or under per night?

More detail please! How many per room, and if more than 1, is £30 (€50) per person or per room. Some great value hotels if you are getting a room for more than one person. Otherwise, try the Golden Pages for a list of B&B's.
Great value hotels include Bewleys, Ballsbridge (€99 per room per night), Bewleys Newlands Cross and Dublin Airport (€79, pay the extra for Ballsbridge, Location, Location, Location), Travel Lodges, and Jurys Inns.

Dublin's v expensive. Glenogra's very nice (on the DART line and about 75 Euro for a night for one person). The Shelbourne Townhouse is pretty cheap and I think it's really fine if you're just looking for a bed and a shower: http://www.shelbournetownhouse.com/. The bus service in this are is horribly bad, but the DART is pretty close. I don't know anything about B&Bs on the noth side, but I'm not sure they'll be much cheaper. Take a look at the Failte Ireland website (http://www.discoverireland.ie/) and maybe actually email them as they can be very helpful.

We recently stayed at the ABC Guesthouse in Dublin for about €35 pp (€70 per room) and it was great. Not in the city center, but an easy bus ride with a very convenient bus stop. You can google ABC Guesthouse and find out more details.
mysterychix
You are spoiled for choice at this time of year and should be able to get good rates especially for three or more night stays (multiple night reduction) Importantly you should still book as far ahead of your trip as possible to get the best deals. Many choices on Lonely Planet's Dublin section. Contact the official Dublin Tourism Centre or an Official Dublin Tour Operator for a list of accommodations in the area of Dublin you require and email/telephone directly with the Manager or Owner,who will hopefully be present at check - in.
Above all enjoy Dublin - it's waiting to enjoy you.