The original inhabitants of Ireland were hunters and gatherers - they came from the European continent around 8000 BC.

An excerpt from the article 39 facts about Ireland

The remains of these people are Neolithic tombs, still visible in the Irish landscape, such as Newgrange - one of the largest corridor graves built by humans around 3200 BC (600 years older than the Great Pyramid of Giza), belonging to the complex called the Palace of the Boyne. It is the most famous of all Irish prehistoric sites.