I've been maining irelia season 5 and I have a few pieces of advice for late game. If you are against a team comp that has two or more big tanks I highly recommend getting tri force and botrk, the % Damage and attack speed are the stats that irelia needs to blow up anyone on the map 1v1. Clearly the other 4 items you build should always be tank items; Randuins, spirit Visage and GA should be the items you build. Against a full AD comp clearly you should not build the spirit Visage, switch it for thornmail or frozen heart.

Anyways specifically for late game team fights even with only a trinity force you can still 1v1 any squishy carry no problem, so with your dive, stun, true damage, ult combo you need to come from the side of the fight and jump onto their adc preferably. If you can manage to kill the adc you've done your job and are already ahead, without his damage on the rest of your team they should be able to clean up. Now If your team is behind I like to make use of her amazing movement speed with trinity force to split push and force 1 or 2 of them top side. I find that with her move speed and q dash it is quite easy to get out of sticky situations and when you consistently bring people to your lane it affords your team time to farm and\or push objectives. If you yourself are having a straight fail game and are way way way behind you need to make the decisions in your mind that all you are going to do is peel for your carries.

If you cannot blow anyone up and keep dying when you split push you are basically relegated to support status but it's not a all lost, just stun anyone who dives, put as much true damage into their face as you can and make sure to know when you need to fall back. Irelia In my opinion is one of the best clean up artists in the entire game. In a team fight save your q for anyone that gets low, when they start backing out q to a low minion, q to the champ that's running away (make sure they die when you do this) then you can q back to their front line so you aren't caught in the back. I'm only silver but do have 75% win ratio on irelia over 30 ranked games this season.