When would I rush youmuu's? Basically never an item to rush I have never evne really hear about rushing ghostblade but it has been a decent item for later on in boosting overall damage aginst squishy targets since all but E is physical damage or true damage, It is however not really better then Maw in most cases currently.
When would I rush sheen? If you are considering rushing trinity instead of rageblade and are against a squsihy target Or you want a lot of damage and have enough for a sheen but not a phage. Basically sheen is the burst damage item
When would I rush phage? Same situation as Sheen but when the enemy is kind tanky or if you have to back and only have enough gold for phage parts but not enough for a sheen. In other words the sustained damage item.
When would I rush rageblade? Generally the best item to rush currently since it is more damage then trinity in Irelia's full combo for 1000 less gold. IF you have the gold for a piece of rageblade get it instead of a longsword and/or ruby crystal if you want to get the biggest damage boost in laning.
Should I ever get berserker greaves? If you are facerolling it can be justified but otherwise ask yourself if any enemies are really a threat and build to contend with them.
Should I ever get hextech gunblade? IF you are looking for godly sustain gunblade is currently justified on Irelia after a rageblade.
And as far as skill order goes is it: usually eqwwwrwewereeqqrqq against melee and for ranged qewwwrwewereeqqrqq
Max W if they're easy to autoattack? If they are easy to auto attack and you are likely to do more then just a single auto in a trade to the enemy. Also ideal for sustain overall
Max E vs everything right? Maxing E is more of a case of knowing you will likely be having issues disengaging or against a squishy who you can out damage in an extended trade.
I play Irelia mid what is a good item to consider against burst champs? Hexdrinker is really good for stopping champs like Leblanc and Syndra from blowing you up.