I need help identifying a Celtic fantasy/romance story involving a Lord and a young girl

Question

I'm having trouble remembering the title of a book a read when I was in middle school but I'm compelled to track it down again years later. Here are the details I remember : The main character was a young girl. She comes to stay with this Lord who is her senior by several years and they start slowly falling for each other. They realize they are meant for each other in a scene where they jump over a bonfire on midsummer. They break peat bricks open and the filaments match the hair of the other. The tradition was that the color of the filaments in the peat brick would be the hair color of their soulmate. The hair colors were flaming red and bright silver. I believe she is taken at one point and imprisoned in some catacombs as well. The Lord was a sailor too, which was a focal point of the story. And they made quite a deal about the castle he presided over and mentioned many out lying building that I can't quite seem to remember.

Those details stick out to me clearly but beyond that I can't remember anything and it is unreasonably frustrating. Please help if you can. And thank you for your consideration in advance.

Answer 1

This is The Folk Keeper by Franny Billingsley (2001).

From page 85:

It was laughing Sir Andrew of the smouldering shoe who finally explained it to me. Each unmarried person holds a half-burned brick of peat against his heart for no fewer than seven minutes. "When you break it in two, Corin, the color of the strands that hold the peat together will match the hair of the lucky lady you are to marry."

And page 92:

I am back, staring into the twilight mirror. It is all silliness, and wouldn't Finian laugh if he knew that the strands that bind my peat are dark red.

Corin is, in fact, a 15-year-old girl named Corinna who is in disguise as a boy in order to work as a Folk Keeper (who can keep pesky supernatural creatures at bay). Based on this ability, the elderly Lord Merton of Marblehaught Park has summoned her to his estate to deal with the Folk that are ruining crops, etc. It ends up being far more than a job: she ends up gradually piecing together her own backstory and origin, little by little. In the process, on top of falling for the aforementioned Finian (Lord Merton's stepson), Corinna lets her short silver hair grow out and discovers that it gives her access to some supernatural abilities linked to the sea. However, it also becomes increasingly apparent that one Sir Edward really wants the estate for himself, and is willing to do anything in order to get it. Corinna ends up trapped in a system of tunnels under the estate for some time, but that might be a blessing in disguise, since it gives her a chance to think things through and experiment with her newfound powers.