The Answer opened their midnight-hour SXSW show with a few words. “We’re from Belfast, Northern Ireland, and our business is rock & roll,” crowed singer Cormac Neeson, a hyperactive beanpole with a fierce vibrato in his throat. Then the four hard-blues boys proved they knew their business well with an assault that was heavy Seventies a la UFO and Irish slide-guitar god Rory Gallagher, delivered with smart writing and true-believers swagger. Neeson introduced “Under the Sky” as “swamp music, Irish-style” — he was right. The Answer’s 2006 debut, Rise (Albert Productions), is available only as an import, but it is a robust calling card for their live vibe and for the second album, due this fall.