Mullingar Pewter Mechanical Pocket Watch with Trinity Knot

The leaf-life trinity knot, or triquetra (from the Latin 'three-cornered), was used by early Christians to represent the Holy Trinity, though the symbol has its roots in earlier pre-Christian religions, possibly existing thousands of years beforehand. Being such as versatile symbol, the triquetra has countless interpretations - the number three was an important number in Celtic symbolism, so possible meanings also include the unity of the earth/sea/sky, the three stages of life, or various triple-deities. 

The design came to prominence during the period of 'insular art' in the early middle ages, which combined early Christian and Celtic art. Some of its most famous incarnations are found within the artwork of the Book of Kells. It was popularized again during the Celtic revival of the 19th Century.