Celts, Tuatha de Danann, Firbolg
Celts
Celts.
People who are from Ireland are called Irish.
The people who lived in Nineveh are often referred to as Assyrians, as Nineveh was a prominent city in the ancient Assyrian empire.
Modern people call the language of ancient Greece Ancient Greek.
They would also call them boats.
Ireland is the country. Éire is the name for Ireland in the Irish language, not the name of the language itself. Irish people call it Irish, people outside of Ireland call it Gaelic, and in the language itself it is Gaeilge. Ériu was an ancient goddess, and using her name and the suffix "land", the name Ireland came into being, with Éire being the name in Irish, as mentioned in the Irish Constitution.
People from the Stone Age are often referred to as prehistoric or ancient humans. They lived during a period in history when stone tools were predominantly used for hunting and gathering.
The Vikings.
Yes, its normal to call people by their first name once you have been introduced to them.
civil lawsuits
They are Irish. You could refer to them as Irish, or as an Irishman or Irishwoman.
Northern Ireland is in the UK, and the international dialling code for the UK is +44.
People in Ireland call him Santy, Santa and Santa Claus. Santa Claus comes from Saint Nicholas and can then be shortened to Santa Claus or just Santa or Santy. So people in Ireland use those names, like people in many countries do.