For my daughter's wedding, our florist designed gorgeous bouquets that included a flower called Bells of Ireland. In Wales, brides carried live myrtle and gave a sprig to each bridesmaid which they planted. If it grew, the bridesmaid would marry within the year. If you're planning a more general Celtic celebration, this might be worth considering. Also, when a couple is dancing, the bride can't take both feet off the floor because the fairies will get the upper hand.
Fairies love beautiful things and one of their favorites is a bride.
For the same reason, it's bad luck for a bride to wear green. I've also heard that it's bad luck for anyone to wear green at an Irish wedding - but I think it really only applies to the bride. It's also bad luck for a bride or the groom to sing at their own wedding. If you're a Roman Catholic, one way to make certain that it won't rain is to put a statue of the Infant of Prague outside the church before your ceremony. It was unlucky to marry on a Saturday.