The Triquetra, more commonly known as the Trinity Knot, is a Celtic symbol of ancient origin and one of the earliest symbols of Christianity predating the crucifix by hundreds of years. The triquetra is most simply represented by three interlocking circles. Often the triquetra is found illustrated with three fish in a similar shape. This symbol is theorized to represent a three in one concept similar in concept to the triple spiral. The triquetra could have symbolized earth, air, and water. Another theory is the triquetra symbolized mind, body, and soul.
Because we cannot with certainty know the ancient Celt's views on a soul we can speculate about their views on rebirth. So using the modern day connotation of soul we can conceptually understand the Celtic intention to represent interconnectedness of three parts in one. When Christianity was introduced to Ireland, the Triquetra became known as the Trinity Knot which symbolized the persons in one God: the Father, the Son and the Holy Spirit.