Irish mythology often talks of the Tuatha De Danann, a race of Celtic gods who settled in Ireland long before Christianity took hold. However, much older supernatural races already existed there, namely the Fomorians – or the Fomoire as they are known in Old Irish.
Though their true characteristics are debated, the Fomorians are generally considered to be hideous giants or demons who came from below the sea. Tales of ghostly underwater cities were common in Celtic lore, particularly the French legend of Ys. Despite being rivals, there are several mentions of Fomorians – the deities of chaos and destruction – and Tuath De – the deities of order and fertility – intermarrying, which suggests their relationship was similar to the gods and giants of Norse mythology.