Tree of Life Overview - Baldur's Gate 2
This is the penultimate zone of the entire game and is reached by completing the puzzle in Suldanessellar and traveling through the Elven Palace. Before being able to fight Irenicus you will have to travel around this map and destroy the parasites that are clinging to the Tree of Life. There are a total of 3 parasites and when you find them simply click on it once to make it summon Elemental defenders then click on it again once the Elementals are dead to kill the parasite.
Once all 3 parasites are defeated you can find Irenicus in the south-central portion of the map. It's (obviously) strongly recommended that you save your game before fighting Irenicus and buff up before attacking him. If you want a more detailed explanation of how to beat him check out Chapter 7 of my Walkthrough.
Parasite: Your main objective on this map is going to be visiting each of the 3 Parasite connected to the Tree of Life and destroying them. The first time you click on the Parasite it will summon Elementals to defend it, once they're defeated click on the Parasite again to destroy it. Once all 3 Parasites are destroyed Irenicus will become vulnerable and you'll be able to defeat him.
Irenicus: This is where you'll find Irenicus, the final boss (part 1) of the game. He will ignore you completely until after you've destroyed all 3 Parasites; once all 3 are dead you'll be able to engage him. You have a few ways of dealing with Irenicus, the first and most cheese method is using traps. You can place as many as you want at the feet of Irenicus and potentially even instantly kill him when he activates. Another method of fighting him is running away at the start then summoning Animate Dead/Conjure Animals and sending them down to Irenicus. Irenicus will dispatch them rather quickly but he'll use up a lot of his higher level spells doing so making the fight with him easier. For more strategies and tips for how to play this game and beat difficult bosses check out my BG2 Beginner's Guide.