When Holly goes to Ireland in P.S. I Love You she meets William in the bar who's playing "The Galway Girl", this is the same song that Gerry had played for her. She assumes that this is all part of Gerry's plan and he'd asked William to play it for her.
However, later we learn that William didn't know Holly was "Gerry's Holly", if he didn't know how could he know which song to play? Is it a coincidence? Is he lying?