The history of Ireland dates back to ancient times, with the first settlers arriving around 8000 BC. Over the centuries, various tribes and cultures, including the Celts, shaped the island's identity. By the 5th century AD, Christianity began to spread, leading to the establishment of monasteries and the development of a rich literary tradition. In the 12th century, the Norman invasion marked the beginning of English influence in Ireland. This led to centuries of conflict, culminating in the Irish War of Independence in the early 20th century. In 1922, most of Ireland became the Irish Free State, while Northern Ireland remained part of the United Kingdom.