This Build/Guide was inspired by Reginald currently the highest ranked ELO player in the game.
This Guide will not go into depth too much on how to play Ireila but it will most certainly cover the basics.

Marks - Armor penetration (self explanatory)
Seals - Mp5 gives an early game advantage on harassing, last hitting, slaying, and ganking.
Glyphs - CD reduction is nice regardless what champion you play on and combined with masteries and spirit visage it puts you at 20% CD reduction.
Quints - HP quints also puts you at early game advantage while in lane solo or not.

Items as for items go, i prefer this build over any others for the simple fact that Ireila's strength is her being tanky and being able to do damage and this build really strengthens that feature as well as keeping very good damage. you will be able to have very good damage out put while being able to survive in a team fight.

I prefer to have bladesurge as my first ability to get last hits early and also have very good harass early on to soften up your oppenents. getting your Equiluibrium Strike really provides good combo to Q then E for good damage as well as a slow or stun under a turret etc.. then i chose to max out Hiten Style because its important to stay in lane as long as possible.

Ghost - Great for get away, Ireila has a high natural run speed 400+ after boots and combined with trinity force the 27% puts at almost 550 which combined with your ES is almost guaranteed a get away.

Ignite - Adds good support and damage. and preferred over any of the other summoner abilities.

Pros

Tanky
Very Good Damage
High Survivability
Able to Gain life through Abilities
True Damage
Versatile, Able to solo or lane or even jungle if done properly.

Cons

Tricky to get used to
Damage slightly curves off end game.