A stone (usually erect) with Ogham script carved on it. They are usually monuments dating from the Iron Age with the inscription being a dedication to somebody, probably a memorial. They often simply state a persons name and pedigree in the form of Fred Son Of Roger Son Of Herbert.
Quite often these are re-used Bronze Age (or earlier) standing stones. As they are often associated with ecclesiastical sites and that the writing seems to be dated from the period starting around 300 CE, many think that it was in fact brought to Ireland by the first Christian missionaries, while other still refer to it as the writing of Druids.
Ogham or Ogam writing is an early script based around the Latin alphabet. The characters used to depict letters are formed by one to five stroke either above, below or through a baseline. When written down this line is horizontal, but when carved on to an ogham stone it is usually one of the vertical edges that is used for this purpose.