I've just dropped this in here so if it's a bit out of place, sorry. I've always thought the song was about how you spot someone and instantly fall in love with them, knowing you will never get to know them as they are complete strangers. Hence the " snare " and unhappiness that he knows will follow the brief glimpse of this beautiful woman.
          The singer Dido has claimed the woman in question is an aunt of her fathers and she has a song called " Grafton St. " on her latest album. She was a student nurse from around the way and went on to be considered one of the most beautiful women in Ireland.