Celtic tattoos were a likely sight among Celtic warriors.
Much like the Picts (Latin root word: Picti meaning "painted ones"), who tattooed their warriors as a form of intimidation against their enemies, Celts likely adopted the same war tactic of the time.
Battle among the Celts was considered the very highest honor. The Celts engaged in battle bare-chested, or even naked - further reinforcing a stance of intimidation against their foes. Additionally, Celtic warriors would fashion their hair with bright dies (from flower pastes), and typically manipulate the hair in tall spikes around the head (think Celtic punk rock).
The Woad plant (Isatis tinctoria) was used to perform the tattooing, as it is a hardy biennial plant native to northern Europe and the British Isles. Woad is the source of a blue dye chemical, indigotin, that is also produced by the much more potent indigo plant (indigenous of the sub-tropics).
To perform Celtic tattoos, the leaves of the Woad plant are harvested and dried. The dried leaves are then boiled and strained, and boiled again creating a viscous end product. This Woad paste is then tapped into the skin with needle like implements - forcing the indigo stain under the skin layers, creating the indelible design. Illustrations of woad tattoos on Picts show the La Tene designs as the body modifications of choice.