You should be able to find someone with a spare meet/greet in a suitable location; I just hope you're not disappointed. Meet/greets are not set-up for much conversation any more. You get to see all four girls, but you get a fast pose/picture/thank-you-very-much, and the management keeps the line moving.
HOWEVER, you may get other opportunities to meet Mairead. With the upcoming release of a new CD/DVD, there is a chance that Celtic Woman will do some in-store signings, monitor Barnes/Noble -- Chicago is one of their favorite cities. ALSO, PBS fund-raising campaigns always feature Celtic Woman shows, and sometimes the girls are in-studio guests. Consider being a volunteer during the CW show -- and some of the jobs don't involve being a call-receiver.
It's amazing how big Luck is in meeting the girls. If you drive by the concert venue, sometimes you see them walking around. I saw Mairead in Atlanta walking down the street with her husband. AND another time at the Atlanta airport at the STARBUCKS, there was Mairead waiting for her plane. She was soooo good to us and spent a few minutes talking and taking pictures (I'm not trying to make you jealous, just demonstrating how important Luck can be).
Good Luck! 7 years is long enough to wait.