@Séimí Mac Síomón The Abhartach legend is awesome. I found a few interesting things. 
In that part of the world there are constant references to a time when the veil between the living and dead was thin. Its in many different stories. Something had to be going on there. 
I found some stuff about the sword the druid used to kill him. The Yew wood is significant and might be the reason we had 'stake through the heart' associated with modern vampires. Check this out- 
1- Yew: Known as 'the witches wood' and the wood of death, it draws off the element of water. Its a powerful symbol of death and reincarnation. Often yew is associated with sorcery and dark magic. Yew is excellent for rituals and divination. Yew trees were planted in graveyards to protect the spirits of the dead. Working with Yew can reveal the past, bringing wisdom and renewed spiritual strength. Yew: Yew wood is heavy, very hard and firm, tough and elastic even though it is actually a softwood. It is not susceptible to shrinkage and has good stability. The wood is resistant to weather and not susceptible to fungal or insect infestation.
2- The Celts will also have been familiar with the toxicity of the tree’s needles in particular. This may have further contributed to its connections with death. Shakespeare was familiar with these qualities when he had Macbeth concoct a poisonous brew. The deadly drink included “slips of yew, silvered in the moon’s eclipse”.
https://image.nostr.build/efa1f1c4e239eeee877bf3b728a069fbc6b4d841ef0edf0514052702fa0cb67b.png 

 The Yew is an evergreen tree as well, often planted in graveyards as a symbol of everlasting life.
It's certainly a tree that has captured peoples imaginations.
English longbows were made from Yew if memory serves. 

 A local tradition has it that a branch of Yew, with a message tied to it, when burnt on Samhain, can be read by the dead.