While Celtic designs come predominantly from the peoples of the British Isles, Celtic society has its roots in Austria, Germany, and France, perhaps going back as early as the 2nd millenium BCE. Indeed, the Celts are but one of the Indo-European groups who occupied Europe, Persia, the subcontinent of India, and some other parts of Asia.
In tattoo imagery, though, it is the artwork of the Celts in Ireland, Highland Scotland, the Isle of Man, Wales, Brittany, and Britain that dominates–intricate and interwoven knotwork animals and letters are the mainstay, as is the Celtic Cross or Shamrock.
The fantastic illuminated manuscripts that have survived began to be produced in the Celtic west sometime after about 500 CE, with religion surviving in the monasteries and artist monks laboring to produce ornate and lavishly illustrated Christian manuscripts such as the Book of Kells, the Lindisfarne Gospels, or the Book of Durrow. Much of modern Celtic tattoo art is modelled on ornamentation from these manuscripts.
But when we speak of Celtic tattoos, there are two subjects to which we can refer: ancient body decoration among the historical Celtic people and modern tattoo symbols inspired from their artwork and the artwork of their descendants (such as the Beard Pullers, Claddagh, Dragon, Peacock, Tree of Life, and Triquetra).