Need advice??

I am applying for a new job that I desperately want. I just got a call that I will be contacted for an interview in a couple of weeks. But here's the problem. One of my family members has a chance to go to Ireland for a week and she's invited me. Going to Ireland is like a once in a lifetime thing for me but I also need this new job really bad. How do I incorporate both? Do I tell them in the interview that I might be going on vacation or after I get hired? (which i'm pretty sure since I have a friend working there) Some people are telling me to say I won the trip to Ireland but If i get the job I don't want to lie to them. I'm afraid if I tell them right away that I may be going to Ireland they won't hire me. What to do??