Celtic Tree of Life (Crann Bethadh)

Trees were sacred to the Celts. Their roots lay in the underworld, their trunks in the earthworld and their branches reached into the skyworld, physically uniting heaven and earth and making Earth Goddess and Sky God one. Trees were also a source of basic sustenance providing food from their fruits, boards to make shelters and fuel for cooking and warmth. Wood from sacred trees was held to have magical properties which was reflected in the Celtic Ogham alphabet, wherein each letter represents a particular sacred tree (modern Ogham divination is based on the uses and importance of these sacred trees to the Celtic people). Some trees provided food, some wood for making hunting weapons; others were sacred to the fairy-folk or to the Gods. In Celtic creation stories, trees were the ancestors of mankind, elder beings of wisdom who provided the alphabet, the calendar, and entrance to the realms of the Gods.
The most sacred tree of all was the Oak tree, which represented the
axis mundi, the center of the world. The Celtic name for oak, daur, is the origin of the word door- the root of the oak was literally the doorway to the Otherworld, the realm of Fairy. The word Druid, the name of the Celtic Priestly class, is compounded from the words for oak and wise- a Druid was one who was "Oak Wise," meaning learned in Tree magic and guardian or the doorway.
The lore of trees persists as a vital part of Celtic myth and folklore. Countless Irish legends revolve around trees. One could fall asleep next to a particular tree and awake in the fairy realm. In Celtic legends of the Gods, trees guard sacred wells and provide healing, shelter, and wisdom. Trees carried messages to the other realm, and conferred blessings- to this day, trees can be seen in the Irish countryside festooned with ribbons and pleas for favors, love, healing,
and prosperity.