That battle is actually one of the easier ones in tactics/ascension. Personally I found Illyich, Kuroisan, Red Badge, Final Ascension, improved mindflayers... well, a lot of the battles harder than that. Improved Mindflayers was the meanest, I did them absolutely last of SoA and I had such a goddamn hard time, eventhough I had planetars and stuff in my arsenal.
The final Irenicus fight is actually, if I recall correctly, 3 battles in one. First it's against all of those, then you drop all your items (which you could - and should - just instantly pick up and re-equip) and fight another form, and then there's a third battle. I don't remember exactly what it's like, and it did take me a few reloads, but again, so did a lot of the battles.
Also, the first fight is the most difficult. Once I beat that it only took 1-2 reloads to beat the other two stages.
First of all, there's no turning back, so get all the items and the experience you can. You should also have some powerhouse classes, such as an inquisitor and a sorceress. And do WK for the items/experience.
I don't recall exactly what I did in the fight itself, but as soon as I gained some levels my sorceress was running around imprisoning everything in sight. I'm pretty sure I took out 1-3 of them before the fight even started with time stop+imprisonment.
If you have played difficult mods before, this one shouldn't be that hard. You hardly ever hear someone talk about it as being up there with the other insane mods. As long as you know how to utilize most of the powerful moves in your arsenal (planetars, mordenkainens, contengencies, inquisitor dispel, hardiness, imp haste, pro-mw, spell immunity, etc etc etc) you should be able to do this quite easy.