At the start of the Damage Step, if this card attacked a face-down Defense Position monster: Destroy that monster. You must control a face-up "Six Samurai" monster with a different name to activate and to resolve this effect. If this card would be destroyed, you can destroy another face-up "Six Samurai" monster you control instead.
Best card to use Irou's effect is to use Floodgate to flip your opponent's summoned monster to face down. This lets him destroy most monster, but you will still need to have another face-up Six Samurai monster with a different name on your side of the field.
All of the Six Samurai monsters are warrior-type monsters, so using The A. Forces will help boost their attack, which can be devastating if you can swarm the field. Swiftstrike Armor can be a little slow, but it can potentially let you get an extra summon during your turns, allowing you to get 2 monsters out on the field. It can also be used with Reload since all of the monsters you get after using Reload can be special summoned with Swifstrike Armor.
You can use Powerful Rebirth to bring back a Six Samurai monster with a different name during battle step when attacking a face down monster with Irou to bypass cards like Sphere Kuriboh. Legendary Ebon Steed is a useful card to protect your monsters and give it some attack increase.