A portal in a rock or barn??

This is going to be a long shot because I can't remember everything! I'm not sure if the book is considered young adult or science fiction/fantasy. I believe the book took place in Scotland or Ireland. A young woman finds a portal (possibly in a rock or a barn) and is able to go to a different dimension. She was able to do this after meeting an older gentleman and his dog on a beach. The dog's name was Loughery or something very close to that. While in the other dimension the young woman attempts to save a beast who is trapped in a cellar or somewhere underground. She also finds a world on a beach which looks like a huge crumbling city. I think at the end of the book the young woman's step-mother (I think) tries to kill her. I feel like this scene took place in a barn. I sincerely hope someone can help! I read the book sometime in the early 2000s and it may be part of a trilogy.

Could be Prospero's Children by Jan Siegel.

YES!!!! I have been thinking about this book for years...thank you!!!