Celtic tattoos were very much a part of the Celtic warrior culture. The Picts (named after the latin Picti meaning “painted ones”) would adorn themselves with tattoos to make themselves a more formidable sight for their enemies. The other Celtic tribes would have adopted a similar style.
They fought with little or no armor, with dyed and spiked hair. Combine this with the tattoos and the fact they’re a big hairy barbarian and you have something that you definitely would not want to mess with.
The tattoos were carried out using the Woad plant. The leaves were boiled, strained and reduced into a sort of primitive ink. This was then tapped into the skin using needle like objects creating a blue tattoo.
There were various types of imagery used for these tattoos. Rune like symbols were common among the warriors as they believed the symbols to carry various spell like powers when tattooed such as strength or protection.
Celtic knots were also commonplace and would often be tattooed on the arms and chest area.
Animal designs were also popular and carried much meaning. Animal designs were probably also worn by the warriors as they were believed to symbolize the strengths of the various animals.
There is a host of other Celtic designs such as crosses, La Tene, trinities, etc. These symbols would each have their own meaning, probably to reflect nature or the elements.
There has been a lot of research done on the various meanings of certain symbols and designs, far too much to go into in this post. If you are considering a Celtic design I would definitely recommend either getting a specialist book on the subject or doing some thorough research online before committing it to skin.