Can't remember the title of a book about a girl, a stable boy, and the ghost of another stable boy and has the song O'er the Water to Charlie

So here's what I remember about the book. Bear in mind it was the early to mid 90's when I read it so some of the details could be wrong. The main character is a teenage girl (possibly American), I think it's set in Ireland or Scotland. The girl is on a rural farm with horses (I think one of them was a black stallion) and there's a stable boy she's interested in.

The song O'er the water to Charlie is referenced multiple times but the primary part is the middle of the song "I swear by moon and stars sae bright and sun that glances early. If I had twenty thousand lives I'd gie them all to Charlie. We'll o'er the water, we'll o'er the sea. We'll o'er the water to Charlie. Come weal and come woe, we'll gather and go and live or die wi Charlie."

It also involves a girl named Charlotte/Charlie and boy who were in love a long time prior to the beginning of the story and they died and iirc their spirits occasionally possess the FMC and the stable boy. The book ends with the stable catching on fire and like in the past, the stable boy dies trying to free the horses.

The cover was of the main character standing outside in all black, her hair was black and blowing in the wind while she looks off to the side