I appologize for taking so long on this ive been asked to post, but havent had time since new expansion has been out Ive been super busy. Infact this has been my favorite expansion in ages thus far.
Anyway, as someone has mentioned already, at the start of the event you have 4 portal attendants. They activate every 2 % of Irrissa's life till 92% (I.E. 98 96 94 92). They spawn from left to right in the order of 1-3-2-4. They absolutely must be burned down asap, or else you face Irrissa powering up, his melee hits. (to 10-12k or more).
The next phase:
Starting at 90% Irrissa starts spawning clones of himself, that need to be killed.
90% = 1 Clone
70% = 1 Clone
60% = 2 Clones (IE 3 Total Irrissa up at once and does hurt if not ready)
40% = 1 Clone
10% = 2 Clones.
Once at the 10% mark just burn the real down then handle the fakes after. The raid isnt over till both fakes are dead even if real is dead first.
There is a nasty dot as well that is done from emote based. I for the life of me do not know the exact text of the emote that the single individual gets who is being targeted on. Everyone else see's a generic emote. Anyway someone else can give the trigger text Im sure. This person who is emoted needs to get away from the raids and take the dd/dot. If they stay in the raid area everyone gets blasted with it and its not good. Irrissa will phase in and out of the plane (Disappear and Respawn) during this emote, the good thing of this is when he respawns the tank who had the agro before the despawn is auto agroed/summoned if not there.
Thats about all there is to this event really. Dealing with the 2 fakes and real at the same time imo is the only challenge of it. At lvl 80 your guilds should have enough dps at this point (if not slacking) to kill attendants in the time frame given before penalties hit.
Im sure if I messed up something in here someone will take the time to correct it now and you will have your strat, but you have the general idea of what to expect now.