Nice thread we've got here. Hm, I'd love to hear Don't Take Your Guns To Town done by Elvis. It's a Johnny Cash song and western/Irish themes fitted Elvis well enough (the song is quite cowboy but the structure is from an Irish folk song).
And if only he could have done complete versions of Roy Orbison's Running Scared and Only The Lonely. Maybe Elvis could have done U2's ' One' as well.