I read this while growing up and loved it. A boy and girl in rural Ireland find that the wise little grey donkey Long Ears can take them into adventures with strange people from the Celtic past. It is great fun and you absorb a lot of misty myths and details about rural life before telephones and plentiful cars.
Recommended for children or the young at heart. Patricia Lynch is a wonderful storyteller with a beautiful style of writing. Her people and animals become very real. There are more stories of the turf-cutter's donkey and a separate book called The King of The Tinkers.
For a more modern take on a very similar story read The Hounds of the Morrigan by Pat O'Shea, published 1985.