The triquetra has been found on runestones in Northern Europe and on early Germanic coins. It presumably had pagan religious meaning and it bears a resemblance to the Valknut, a symbol associated with Odin. The triquetra is often found in Insular art, most notably metal work and in illuminated manuscripts like the Book of Kells. It is also found in similar artwork on Celtic crosses and slabs from the early Christian period.
Pagan Celtic Sleep Blessing (This is the original image. There is a version floating around the internet that is a crude alteration and doesn't have the artist's copyright on it. Please share this version and help spread the word about the original artist's work.)