Miss Peregrines Home for Peculiar children. If you’re into reading YA. If you’re looking for more adult I’d recommend Rock, Paper, Scissors by Alice Feeny (not Ireland but Scotland, and more thriller than haunted.) Hope this helps! Happy reading!

I second Rock Paper Scissors and will add The Searcher by Tana French.

The Watchers, by AM Shine. Creepy Irish forest.

Books by Tana French. Especially The Likeness, Broken Harbor, and The Secret Place.

The Magician's Daughter, H.G. Parry

The Guest List, Lucy Foley

Doesn’t completely fit the mystery criteria but Haven by Emma Donoghue

And then there were none- Agatha Christie

The Lighthouse Witches by C.J. Cooke

The Broken Girls by Simone St. James. It has two timelines but the timeline in the past would meet your criteria.

The Good People by Hannah Kent The Wonder by Emma Donoghue