Ogham (OH-yam) is believed to have been devised by the Irish somewhere between the first and third centuries AD. Surviving examples place its' primary use to the Christian period, however, some Celtic scholars believe it to be of far greater antiquity.
Existing examples suggest that Ogham was used primarily on grave and boundry markers. Indeed, most inscriptions read "so and so, son of so and so, son of so and so" and so on. Evidence exists, however, supporting its' use by Druids for recording tales, histories, poetry, geneologies, and the like. Bards are thought to have carried a Táball-Lorg or 'Poets Staff'. This would have been a staff comprised of several wooden wands, fastened at the bottom so as to open into a fan shape. It was on these wands that the poets would have inscribed their tales.
The name Ogham or Ogam (Ohm) was derived from that of the Celtic god of literature and eloquence, Ogma, who is credited with its' invention. The letters are constructed using a combination of lines placed adjacent to or crossing a midline. An individual letter may contain from one to five vertical or angled strokes. Vowels were sometimes described as a combination of dots. The midline was, most often, the edge of the object on which the inscription was carved. Ogham is read from top to bottom, left to right.
In keeping with Druidic concepts, each of the Ogham's twenty letters bears the name of a tree. A-Ailim (Elm), B-Bithe (Birch), C-Coll (Hazel), for example. This is not surprising until it is realized that not all of the twenty plants of the Ogham were found in the post-Christian Celtic world of the British Isles. This fact would seem to lend some credence to the theory that Ogham predates the first century AD.
There are 369 verified examples of Ogham writing surviving today. These exist in the form of gallán (standing stones) concentrated in Ireland but scattered across Scotland, the Isle of Man, South Wales, Devonshire, and as far afield as Silchester (the ancient Roman city of Calleva Attrebatum). Similar markings, dating to 500 BC, have been found on standing stones in Spain and Portugal. It is from this area of the Iberian Peninsula that the Celts who colonized Ireland may have come. The discovery of similar carvings in the state of West Virginia in the United States, has caused some speculation that the Celts may have come to the New World as early as 100 BC.