Ogam was the script used in pre-Christian Ireland. Its creation is credited to the god Ogma, who had certain traits in common with Hercules. Ogam was first used for inscription on upright stones, with the letters being composed of notches or grooves along an edge. In later times it was written horizontally. It was often used for magical purposes, with individual letters acting as distinct charms.
Robert Graves, in his seminal work The White Goddess, proposes the ancient correspondences of trees to the Irish Ogam alphabet. I have taken the liberty of reproducing the table below, but you really should track down a copy of this fine book and get away from the computer for a little while.