One of the most ancient of Irish symbols, the triskele (or Triskelion) can be found on the kerbstones of Newgrange which date back to Neolithic times around 3200 BC.
The triskele is pre-Celtic in design, dating back to before the Celts settled in Ireland, but it was incorporated into Celtic culture and frequently appears in Celtic artwork. The symbol is thought to represent continuous movement or continuously moving forward.
The fact that the triskele consists of three spirals, sometimes three legs, adheres to the Celtic belief in the triad, that everything happens in threes; past, present, future: mother, father, child; body, mind, spirit. This would have made it fascinating to the Celts and easy to adopt into their culture.
Centuries later when the Christian church came to Ireland during the 5th century, they also adopted the triskele symbol using it as a tool for teaching the Holy Trinity.