Under Darkening Skies (en Inglés) - K. Orme

Morrigan King is no longer the little girl with romantic notions of the past. But now, after years of masking those notions, she's gotten stuck in a rut of a comfortable cycle. After her grandmother urges her to travel while she can, and an unsavory coworker frays her last nerve, Morrigan takes the leap to go on a trip to County Clare, Ireland, where her family hails from. Now, she can revisit her old passion: researching the family legend of a mysterious relation who fell in love with a highwayman who had been murdered.

Upon arriving at the Dromoland Castle, where she is staying, she meets Darragh O'Shea, a young man local to County Clare with a mystery of his own.

After a ghost tour gone wrong, Morrigan finds out Darragh was a murdered highwayman, back some 300 years ago, with a wife who disappeared from Ireland after his death.

When they get no answers to any of their questions in the present, Morrigan realizes the past is the only way to figure out who her mystery relative & Darragh's wife was, but also who murdered Darragh O'Shea, The Whispering Highwayman.