[Faceman]

might I add a bit of cheese?

The easiest tactic to beat Irenicus with even a low-level party is the following:
memorize a lot of summoning spells, some haste and some other party improvement stuff --
summon some pets before opening the gate - cast haste on your party -
once the fight starts use ther pets to keep the demons busy and retreat down the staircases 2,3 or 4 (as in Dundee's post) -
Now wait for Irenicus spell protections to wear off and send in some of the weaker pets every now and then to make sure he uses up all his high level spells on them - once he is down to the spell level you feel able to face --> charge! (but beware of his Magic Missiles, they may be lvl1 but they are among his best attacks - so if you can't take the heat better summon cannon fodder until he is spellless (love that word))
use haste, improved haste and lots of improvement spells before charging with everything you've got. when he's down to badly injured or Near Death use all the time stop scrolls you have with your strongest (referring to melee) wizard and just hit away at him while tim is freezing, thus hindering him to regenerate.