Irvine Kinneas, the gunman from Galbadia Garden, was assigned to help the SeeDs with the assassination of the sorceress Edea in Deling city. He was known as the best shooter in the garden. And probably as the most flirtatious guy, too. Irvine is convinced that he's god's gift to women, but inside he's serious and sensitive which is why he doesn't work well under pressure... and he uses "alrighty" continously, too. But hey, he has a cowboy hat. No man who wears a cowboy hat can be a bad man.
Just like Quistis, Irvine uses a long range weapon, the Valiant, which comes in handy when fighting enemies which like to counterattack. His limit break "Shot" can be very useful if you aim properly and use strong ammo, however you have to watch out that you don't run out of it. It is recommendable to convert items to ammo instead of buying it to save the money for more important things.
Personally, I can't stand Irvine's limit break because you run out of ammo too quickly. It doesn't really pay off either; again, for damage you should use Renzokuken. I would use Irvine as a healer or general "ability or GF- user".