Buff, Slayer, Repeat, Buff, Slayer, Repeat.
Also, if you can get away with it, cast a Time Stop(Spell or Scroll) and wail away on him Slayerized. Also, auto-pausing is your friend. Open the flood gates and let loose the many, many pausings. This will give you the needed time to wage your battle step-by-step, and possibly get any needed spells cast, or at least about to be cast.
And don't be afraid to let your Slayer go it alone for a bit to let your casters get off thier spells. It is very tough, and if hasted, is incredibly fast.
Also, when you kill Irenicus the first time think of this as something of a breather. I'm sure you can't rest, but cast some healing on yourself, and drink potions like mad before you take him on again, and get off all your buffs. Fortunately, I've never encountered any additional minions of his on the second fight because I kicked his butt right-quick. You might also thin kabout summoning your own minions. Mordekainen's Sword comes to mind. Nearly invincible, powerful, and oh so distracting.
Another good tactic for the second fight is see if you can't hit him with a few Melf's Acid Arrows and get him acidized and unable to cast anything successfully.
Then again, I don't use Mods, so you can ignore all that I just put down as the ramblings of a know-it-all-bastard.
