Rory Gallagher was an amazingly gifted & versatile player, with a great ear for tone, and the live musical dynamic. A great singer, too. Not only could he rock out with his trademark Stratocaster tone, but he was an accomplished steel slide, acoustic guitar, and mandolin player, and featured these instruments throughout his musical catalog. As others have mentioned, he also played with a very talented musicians, some of who have already been mentioned. His line-up on the Irish Tour release I thought was especially hot: Gerry McAvoy (bass), Lou Martin (keys), and Ron de'Ath (drums).
I recently acquired the DVD companion to the Irish Tour release, which was shot as a television documentary. It is an absolute must-have for any true Rory fan, and features a lot of great live footage, including an incendiary performance of the Gallagher rock standard "Walk On Hot Coals", one of my favorite songs of his.
Personally speaking, I don't think you could go wrong getting any of his albums / CD's. I own (4) of his live albums (Irish Tour, European Tour, Stage Struck, Meeting With The G-Man), and they are all loaded with fantastic playing and incredible songs. The two studio releases I have are "Rory Gallagher" (debut LP), and "Against The Grain". Really, I can't say that I prefer any one over the other, as each one is unique, and very good in its own right. But his live stuff, I feel, is where he really shines. As such, if you were looking to add another Rory CD to your collection, then "Irish Tour" would be my recommendation. It has a fantastic live version of "A Million Miles Away" and "Tatoo'ed Lady". Rory also does a killer acoustic slide cover of Tony Joe White's "As The Crow Flies", which alone is worth the price of the CD. "Walk On Hot Coals" is as hot as it gets, and the four minute guitar outro on that song is a testament to Rory's incredible skills as a guitarist, and as a live performer of the highest caliber.