From what I've read there was no Fifth International Brigade, but as someone pointed out above the IBs started at 11. The Spanish song 'Viva La Quinta Brigada' is about the Spanish 5th Brigade, the Christy Moore song is about the 15th International Brigade. At some point after he recorded the original, erroneous, version, Christy realised the mistake and changed to Quince (15), which he has stuck to ever since.