I always took this scene to be a wonderful coincidence. The whole point of Gerry's many messages to Holly is to push her towards the possibilities of the future. Whilst he had a lot planned, I like to think that he hoped that by moving on with her life, opening herself up to possibilities and meeting new people, she would fall in love again.
So whilst the playing of The Galway Girl is, in my view, coincidental, it could have been anything that first caught her attention. It could have been a made who looked like Gerry, or who had the same job, or who liked the same sports team. Anything could have been a reference - especially since she was looking for them.
But as we later realised, it really did seem to be a coincidence, showing that by opening herself up to new possibilities, Holly was allowing herself the chance to move on - which she eventually did.