When the Tuatha, also known as the People of the Light, arrived in Ireland, coming through the mists and clouds on airships, they encountered and defeated dark forces present in the land. Their weaponry was magic, primarily shapeshifting and weather magic.
Eventually the Milesians, the Celtic people, also came to Ireland and in another epic battle on the same battlefields near Sligo, they defeated the Tuatha. Although there is much written and said about this final battle, my favorite image comes from poet and author David Whyte. The Tuatha and Milesians were lined up on opposing ridges overlooking the battlefield, the Tuath resplendently dressed with a shining countenance and banners flying. When the Milesians descended to the field for battle, the Tuatha all turned to face the sun and disappeared into the light.
After this defeat it was agreed the Milesians would claim the world above ground and the Tuatha would live within the land itself. A parallel existence in a parallel dimension – a parallel dimension in the landscape of Benbulben, indeed a mountain of great mystery and power.