Though often thought of as an early Celtic symbol, the Triquetra does not appear on any Celtic works until after the Christianization of Ireland, where it could be found Insular art. The symbol also begins to appear on Norse rune stones about this time, and it is entirely possible that the Celtic Triquetra is a stylized version of the Norse Triple Horn found on earlier rune stones. The Vikings would have come across the Triquetra through monastic raids, and possibly viewed it as a new symbol. A lot like a game of telephone, but with raiding parties and more bloodshed.
What is known is that the Triquetra is the only Triskelion developed during the Christian period, and to the early Celtic Christians this was a symbol of the Holy Trinity.
Recently the Triquetra has gained an association with Wicca thanks to the television show "Charmed". The only reason the Triquetra even appeared on the show was because the producers wanted a visual symbol to represent "the craft"; and the Pentacle, the official symbol of Wicca, which was deemed too controversial, with negative overtones. Another reoccurring modern myth is that the Triquetra with the circle is Christian and without is Wiccan or vice versa, both versions can be found in countless places in Insular works like the Book of Kells.