The Celts were a European cultural group first evident in the 7th century B.C. The Romans called them Galli and the Greeks called them Keltoi-- both meaning barbarians. Their maximum expansion was in the 3rd to 5th century B.C., when they occupied much of Europe north of the Alps. The Celts arrived in Britain by the 4th or 5th century B.C. and Ireland by the 2nd or 3rd century B.C., displacing an earlier people who were already on the islands. The Gaels, Gauls, Britons, Irish, and Gallations were all Celtic people.
The Celtic peoples are defined as those groups that spoke a derivative of the Celtic branch of the Indo-European language family. This definition encompasses many tribes which had no concept of nationhood nor any central authority, yet shared many technological, artistic, and philosophical ideas. On the continent, the expanding Roman Empire conquered various Celtic groups. Julius Caesar conducted a successful campaign against the Gauls and as part of that campaign invaded Britain in 54 B.C. but was unsuccessful in conquering the island. Ninety-seven years later the Romans invaded Britain again, pushing the Britons to the west (Wales and Cornwall) and north (Scotland). Hadrian's Wall was built beginning in 120 to protect the Romans from the northern Celtic tribes.
The Romans never occupied Ireland, nor did the Anglo-Saxons who invaded Britain after the Romans withdrew in the 5th century, so Celtic culture survived more strongly in Ireland than elsewhere. Christianity came to Ireland in the 4th century, St. Patrick coming in 432. Many of the Celtic cultural elements integrated with Christianity. The most "religious" aspect of Celtic culture, Druidic practice, diminished, and many say that the Druids were systematically supressed and killed. However, many cultural elements lasted, including ancient oral stories which were recorded by Irish monks in both Irish and Latin.