I've been in pretty much every pub in Derry city centre and other than Wetherspoons your only options for beer that aren't the usuals are Sandinos and the Bound For Boston.
Sandinos sell bottles of various different beers, off the top of my head they have of Erdinger, Schneiderweisse Tap7(yum!) Budvar, Goose Island IPA, Brooklyn Lager, Duvel, Chimay, Ekstra, Coopers Sparkling Ale. They're 'reasonably' priced too.
The Bound for Boston sells Hoegaarden, Staropramen and Leffe on draught though they're not always available.
Wetherspoons to their credit have decent cask ales, there's normally a Hilden one on and they're a bargain too - £1.79 a pint. They apparently are open to suggestions too, must start asking for more Irish casks next time I'm in.