"A Celebration" was a non album single, recorded in 1982, between the October and War LPs. U2 liked it enough to record a video for it, and played in regularly at concerts through 1983. It then fell off the face of the earth -- the band didn't play it or support the song or video, and it did not even appear on any U2 compilation albums until 2004. Note than in the days before digital music, leaving the song off an album was the kiss of death.
The reason for all of the controversy was due to a misinterpretation of the lyrics. The lyrics "I believe in a third world war. I believe in the atomic bomb." were meant to be darkly humorous, but they were apparently taken seriously, particularly in Europe. In response and to avoid any confusion about where they stood, the band pulled the song, making it and the video rare (and therefore ideal for this blog).
The video for "A Celebration" was shot in the Kilmainham Jail in Dublin, and it shows the band in fine early form. (And man, do they look young). The song actually charted in the U.K. at #47, prior to the controversy, but it did not chart in the U.S.
Note that U2's "I Will Follow" was posted on ERV in August, 2014.