Below you will find the Wikipedia
definitions and also pictures of how the symbols are adopted into modern
tattoo themes - many of the tattoo examples below are a combination of
two or more celtic symbols

Celtic Triangles - Triquetra

The triquetra (often, triqueta) is a tripartate symbol composed of three
interlocked vesica pisces, marking the intersection of three circles.
It is most commonly a symbol of the Holy Trinity (Father, son, Holy spirit)
used by the Celtic Christian Church, sometimes stylized as three interlaced
fish:

The triqueta symbol predates Christianity and was likely a Celtic symbol
of the Goddess, and in the North, a symbol of the god Odin. Although it
is often asserted that the triquetra is a symbol of a tripartite goddess,
no such goddess has been identified with the symbol. Similar symbols do
occur in some Norse and Celtic goddess imagery, but most likely represents
the divisions of the animal kingdom and the three domains of earth mentioned
above. Triplicities were common symbols in Celtic myth and legend, one
of the possible reasons Christian beliefs were so easily adopted by the
Celtic people.

The triqueta makes an ideal Christian symbol. It is a perfect representation
of the concept of “three in one” in Christian trinity beliefs,
and incorporates another popular Christian symbol, the fish, in its original
form of the vesica pisces. It is sometimes enclosed within a circle to
emphasize the unity aspect.