We only know the names used by the Romans, Greeks and any other visitors. The people of prehistoric Ireland would not have had a single name for the whole island. There would have been names for individual tribes - either descriptive or based on the name of a charismatic leader. That was also the case for other parts of Britain -eg the Pictii was a name used by the Roman to describe the painted people who apparently painted or tattooed themselves. We have no idea as to what they called themselves.
It was similar in America. Native Americans were called "Indians" because Columbus was trying to get to India by going westwards. He thought that he had been successful and is why we still call the Caribbean islands the West Indies. He died thinking he had found a new route to India. There is a parallel with the tribes of Europe, Britain and Ireland. Think of the Apaches, Sioux, Seminoles, Blackfeet, Delawares, Mohawks etc. They are clearly related and whereas we can say that they are all "Indians" (even though that name is a mistake) we certainly cannot say that all Indians are Apaches.
Similarly, the prehistoric people of Europe were related and most spoke languages derived from a common Indo-European language. The Celts were one tribe originally living in the Alpine region. We can say that the Celts were Europeans, but we cannot say that all Europeans were Celts. We can say with confidence that the Celts never went anywhere near Ireland.
