TheTriple Spiral is a symbol of growth, transition, and expansion. It is also known as the triskele, triskell, and triskelion. Other symbolisms are: Life-Death-Rebirth, the three realms of Land-Sea-Sky, and Mind-Body-Spirit.
The first recorded instance of the Triple Spiral in Ireland is from approximately 3200BC at Newgrange. This actually pre-dates Celts being in Ireland, and they widely adopted it thereafter. Since Celtic traditions were transmitted orally and not written down, we do not know exactly what the Triple Spiral came to signify for the Celts. One theory is that it symbolized union, creation, and rebirth.
One of the coolest things about Newgrange, the site where Ireland’s first Triple Spirals have been found, is that the site is aligned with the shortest day of the year, the Winter Solstice (December 21). On the Winter Solstice, the first light of the rising sun enters the interior of the site through a box-shaped opening in the roof, stretches down a long hallway, and fills an interior room to illuminate carvings on a back wall, including most prominently — you guessed it — a Triple Spiral.
When Christianity spread into Ireland, the triskele took on a new meaning, to symbolize the Christian Holy Trinity: The Father, Son, and Holy Spirit.
Since this design has such ancient roots and continues to have such strong symbolic meaning today to widely disparate groups of people, it has been adopted as a common symbol for Ireland.