Athair is the modern Irish word for 'Father' and 'Olathair' is an old Irish name that ties directly with one very singular deity. Olathair is best translated as 'great' or 'ample' father and there is only one God of Ireland that fits both of those descriptors very comfortably. He is an Dagda.
Before their arrival in Ireland the Tuatha Dé Danann were said to have traveled far and wide, learning all manner of skill, druidry and other occult practices. The power of this tribe was such that their movements were noted in visions of prophecy long before their ships ever graced the skies