To get Irvine from Flo you have to do most of the Queen of Cards Quest. Getting the Ragnorok on disc 4 I find easiest by taking the portal straight to Centra and using the chocobo from the forest surrounded by mountains.
The Ragnorok on disc 4 does make the game incredibly easy, so I always try to get everything on disc 3, thats why I've only killed Ulty once.