The Ogham Alphabet, BeithLuisNuin or the Alphabet of the Trees.

The Ogham is an ancient Celtic alphabet whose origin is unknown. It has been speculated that the letters, consisting of a series of slashes across a central line in groups of up to five, may have originally been codified hand signals devised by the Druids to represent Greek letters. The remaining Ogham inscriptions are on marker stones which date back to the beginning of the Common Era, many of them come from around the 4th century CE, these tend to be either tribal boundary markers or dolmen dedications.

Although there is no physical evidence, there is much reference in the ancient myths to the use of Oghams by cutting them on pieces of wood or bark. These of course would decay and not be left to us. This alphabet can be used to inscribe steel and wood with Celtic names and phrases.

here is an excellent article on wikipedia about the Ogham

here is a link to an excerpt from one of the Ogham texts called Auraicept Na N-eces: The Scholars' Primer.