What kind of blues, and how far back do you want to go? Two of my favorites are Dave Van Ronk (1936–2002) and Chris Smither (b. 1941, still performing, still a great musician).
I want Van Ronk’s instrumental “St. Louis Tickle” put in a loop and played at my memorial. To me it is absolute feelgood music. Small sampling here. Most of his stuff is authentic bluesy old blues, delivered in the unique Van Ronk style, raspy voice and alleycat yowl and all, and great guitar. This particular number is about as upbeat as they come.
I know Chris from the old coffeehouse days in Boston. I have most of his CDs and go to hear him when he’s in the area. His repertoire used to be heavily traditional, but now it is primarily his own compositions, very literate and unpretentious poetry and very much in the vein of the old greats such as Mississippi John Hurt. He is a magician on the acoustic guitar. His website: http://smither.com/
As for bluegrass, the old masters are Flatt & Scruggs, Bill Monroe, and others of their era. Are you perhaps interested in a sampler album?
For Irish, I don’t think you can top the Clancy Brothers, but there are many performers of various styles, from the rowdy jig-inducing foot-stompers to the mellow harp-playing Celtic sopranos. Can you be more specific about what you’re looking for?