The Treasure of Kilvarra by Elizabeth Baldwin Hazelton, 1974.
Christie and Kevin MacAlistaire go to Ireland with their mother to visit a family friend, Padraic O’Flaherty, and their visit turns into a terrifying treasure hunt! Christie is fascinated by the gypsies living nearby and asks to visit them. It’s her birthday, and the oldest gypsy, Sorcha, tells her fortune. She says that Christie is fated to discover a treasure during her visit to Ireland, but there will be great danger involved, and she will need her brother to protect her. Sorcha also gives Christie an amulet filled with holy ashes to help keep her safe.
Christie is eager to find the treasure Sorcha spoke of, although she doesn’t know what kind of treasure she is supposed to find or what kind of danger she will face along the way. Their new friends tell them the history of the region with stories of ancient kings, fairy folk, and haunted castles. Christie and Kevin visit some of these supposedly haunted ruins with their new friend Colum (a gypsy boy) and his pet crow, Ben. Colum tells them the history of the castle ruins and the legends associated with it, and they discover that Christie has the ability to see and hear things that the boys can’t: spirits of the past. Unfortunately, Ben is fascinated by Christie’s amulet and takes it, losing it in a nearby river.
Without the amulet to protect her, Christie continues her search for the treasure. An accidental injury takes her to the place where it is hidden, but it’s a dangerous place. Caught in a terrible storm, the children explore an ancient stone tower. There, Christie sees the ghost of a long-dead monk, gesturing to her, begging her to follow him to the treasure that she seeks. The monk died protecting it, and it’s a very unexpected but wonderful treasure indeed. But, getting out of the place is going to be even more dangerous than getting in.
This is actually the second book in a series about Christie MacAlistaire and her brother Kevin. Unfortunately, I don’t have the first one and haven’t read it. I do know that the first book in the series is called The Haunted Cove. Christie and Kevin spend the summer in a cottage by the sea in Oregon, where they investigate a cove haunted by the ghosts of sailors and a woman who may be a witch.