Personally I use 1/21/8 (getting the movespeed in support) masteries.

Armpen reds, armor yellows, magic resist blues, and movespeed quints.

My typical build vs a balanced team is merc treads, triforce, randuims, force of nature, atmas, rarely get a 6th item due to how expensive FoN is.

Actually you can if you don't hit your Equilibrium Strike right away which you wouldn't anyway since they'll be lower life then you after popping Hiten Style and Bladesurging. Sheen isn't meant to be run by people that pop every ability as soon as its off CD and Irelia is definitely not meant to played in that way either.

Also the only CD that is reduced with leveling is her Bladesurge which should be kept at Rank 1 till the end game.

A phage start is good when you have limited CC on your team and know you'll be focused first and need the survivability,

Starting any of the Prospector's weapons is generally a bad idea for many champs since they don't upgrade to anything and are only useful for the first couple fights. Its good when you have short builds like AS teemo or AS kogmaw but for long builds with a 4000g item, you just can't afford to delay on it. Also a Phage will do pretty much the same thing plus more at start. You don't need your boots till after you either rc or die anyway.

Finally, in regards to rushing Trinity Force. The fact it increases your Sheen procs to 150% of base damage means even with just 70 base damage your doing an additional 105 more damage every 2 seconds. That's not taking into consideration any of the additional AD or movement speed you gain either. Trinity Force is a balanced item that excels between the 10-15 min mark. At level 16+ you start to see a large diminishing return on your damage since AR scales with level as well as by now even the most fragile glass cannons have gotten some form of AR unless your a very rare case of being the only AD and even then they'll usually get some AR for turret damage.

21/9/0 and 9/21/0 depends on team composition, in my opinion.

When team needs a Tanky Initiator, 9/21/0 gives that slight survivability, especially for the first team fight at top. If there is a Singed or Volibear on the team going top, then Irelia can afford to be 21/9/0. It is probably personal preference.

I run AD quintessences, armor penetration, flat armor, flat magic resist, aiming to secure the first fight to set the pace for the rest of the game. As long as bot doesn't lose too soon, the game usually turns out well.

http://img525.imageshack.us/img525/9858/ireliaalmostpenta.jpg

My quadra screenshot, Irelia with 2 healing potions can usually take out 1 or 2. I max W first.

If you have a good team where u got a nasty ad carry like Jayce, Wu etc. Get your 9/21/0 mastery.
You're are a tank in a way too remember that. Your job is to take shots, and defend, and at the same time dish some damage. Your main job is to defend, disrupt caps, and tank some shots.

Meaning with the defence from 9/21/0 and masteries of AR/Level or flat seals, MR per level glyphs, ArP marks, and MS Quints, or a mix of ArP and MS, you can move fast and disrupt cappers, get away from teamfights with speed if needed, and do good damage. Your job isnt to kill people instantly, so 21/9/0 is not needed since your team should consist of a nasty ad carry like Jayce. So GL, sometimes you can use 21/9/0 if you wanna carry a game but its risky all the time, but pays off if you do well!

9/21/0 with ARP quints & marks. armor/magic resist for the other runes.

9/21 with ArPen + armour + MR runes. Yes, 75 true damage is nothing to scoff at, but the majority of her damage will always be physical.

I'd usually argue against 21/9 for Irelia. She's not an assassin; she's a bruiser who excels in jumping onto the enemy squishies and taking them out of the fight. Someone like Talon needs as much instant burst as he can get, but Irelia is more about sustained damage and disruption.