Boy, released October 20, 1980 was U2's debut album. Common themes on this album are the thoughts, almost ramblings, of the frustrations of adolescence, it included the bands first big hit 'I Will Follow", which focus' on the death of Bono's mother when he was 13. It is also believed/perceived to be a religious song affirming the Christian faith of the band, which was finally confirmed in an interview given in 2007.
The final track on the album, the "Song By U2" that you're looking for was inspired by Williams Golding's book "Lord of the Flies" & shares it's name with a chapter in the book... Chapter 7 I believe
This song is also one of U2's very first songs, it evolved from early live sets and is the only song from the first demo's to ever make it on an album.