For tax reasons!
In Ireland you get extra tax free allowance if you're married. hehe!
Also 'defacto partnerships' are not recogonised in Ireland.
Honestly though it was the most logical step for us. We didn't live together beforehand but knew it was either travel first or get married first. So we did the wedding thing to show our commitment to each other and our family and friends.
We knew we'd get married eventually so decided the sooner the better to get the hassle/stress of the day over and done with