The thing with Irelia is she doesn't really have an early, mid, or late game unfortunately. The good thing about Irelia is she is incredibly versatile in how you can build her. A lot of complaints revolve around how many times she has been Nerfed, when she probably just needs a complete overhaul.
Your priority with Irelia should be either jumping the enemy carries or peeling for you carries. With the way you build and your cc this should be relatively easy unless your enemy is really fed. Being that Irelia has no initiate you shouldn't be the first person to fight, you should be sitting back for just a second to assess the situation and determine where you are needed most, as in peeling or killing.
You should go teleport/ignite or teleport/flash, allowing you to split push or counter gank for your carries.
A thing to keep in mind is Irelia is a Bruiser, so building Tanky/Damage items and masteries/runes is the best choice. Go MR Glyphs, Armor Seals, AD Marks or AS Marks and AD Quints or AS Quints seems to be the typical trend with Irelia.
Your early game goals should be boots and Phlage and stinger (Vs AD) or Wit's End (Vs AP). Next you wanna get Frozen Mallet for nice tankiness and the nice slow. Get Zephyr as soon after, giving you the AS and Movement Speed and no one will escape you. Stick to Beserker's for the AS but if you fall behind go Mercury treads or Ninja tabi. Couple other items would be Randuin's Omen, Guardian Angel, atma's impaler, warmog's, Maw of malimortus.
This build is a very tanky/Damage build, if you aren't already building this change and you'll probably see a performance increase. If you are building this, look at how your playing. Are you getting a lot of CS are you overextending are you roaming when you should be. So many things come into play after you've locked in a decent build.