Brilliant fantasy trilogy wanted to read again for ages

I had the book on an old kindle account and have no idea of the details so lost my entire library... awesome:)

The first book was about the son and his dad but his dad was taken by men on horses all dressed up in medieval armour and when they fell off they turned to dust, later explained that they come from another realm and age incredibly slowly but when the touch the floor on our realm they become the age they would be here and because they’re so old they turned to dust. Beyond that I don’t remember much apart from he wakes up in the other place with his shoes missing all the time and it was based on old Irish folklore? I read this about 5 years ago and really want to get the physical copies, can anyone help pleaseeee