The term “Celtic” has been sometimes used in Society blazons to indicate a style of depiction:  e.g., a “Celtic hound” would be a hound as drawn in the Book of Kells.  The term is thus a direction to the artist, not a variant of type, and carries no heraldic difference; indeed, it’s more commonly not blazoned.  Moreover, a charge drawn too strongly in the Kells style may be rendered unidentifiable, and so unacceptable.  Two exceptions are the Celtic cross and the Celtic harp.

Sabia Gunnhild Hunang bears:  Gules, a Celtic hawk statant close reguardant Or.  [The hawk is drawn as in the Lindisfarne Gospels.]