n Celtic art, the motif of two interlocking commas that appear to swirl is a recurrent one which can be traced back to the late 5th century BC. With a view to the much later Chinese symbol, art historians of the La Tène culture refer anachronistically to these clinging pairs as “yin yang”.
Early Celtic yin yangs are typically not treated for themselves alone, but appear as part of larger floral or animal ornament, such as revolving leaves at the bottom of a palmette or stylized tails of seahorses. In the 3rd century BC, a more geometrical style develops in which the yin yang now figures as a principal ornamental motif. It is not clear whether the Celts attributed any symbolic value to the emblem, but in those cases where it is placed prominently, such as on the upper end of a scabbard, its use seems to have been apotropaic.

Celtic yin yang whorl:

Unlike the classic Taoist symbol, the Celtic yin yang whorls consistently lack the element of mutual penetration, and the two halves are not always portrayed in different colors. In keeping with the dynamic nature of Celtic decor which is characterized by a strong predilection for curvilinear lines, the circles often leave an opening, conveying the impression of the interlocked leaves swirling endlessly around their own axis.Sometimes the yin yang motif is also executed in relief.
The popularity of the design with the Celts is attested by the wide range of artifacts adorned with yin yang roundels. These include beaked flagons, helmets, vases, bowls, collars, hand-pins, cross-slabs, brooches and knife blades. While Celtic iconography was gradually replaced by Roman art on the continent, its revival in post-Roman Britain and persistence in Ireland also saw the resurgence of the ancient yin yang motif; the comma-shaped whorls in a triskele layout in the famous 7th century Book of Durrow are a case in point. ~Wikipedia