The Yew is one of the five power trees of the Irish Fire Druids.

It's Ogham representation is Idho (ee-yo). This stands for reincarnation, wisdom, courage and defiance of death. This probably comes from the fact that the yew is one of the only evergreen trees natural to the British Isles. Obviously a tree that lived on throughout the year while others 'slept' for the winter would have been seen as possessing many powers.

Burying thing beneath them would also give them a magical gardian that was there all year.

Their presence in graveyards probably also stems back to the fact of their seeming 'evergreen' powers.

I guess a certain mythology has grown up around Yew trees (no pun intended), because they can be exceptionally long lived. Generally, if old enough, it's about the only tree that can regrow itself from re rooting it's branches (i.e. where the branches reach the ground, they take root, and feed the tree when the heartwood has died and rotted away, leading to an humungous girth, and the appearance of the tree being part of a group or 'grove' for want of a better word).
I think there is a huge on in Hampshire somewhere rumoured to be 3000 years old or something. Not quite as old as the Bristle Cone Pines of North America which some are believed to be upwards of 5000 years +, but fairly old for Europe.
Though having said that, there are Hornbeams in Epping Forest which are around 1000 years old due to prolonged coppicing.