So, I read this book in the mid-nineties (no idea if it was new or not). I can't recall the title (obviously), but relevant plot points:
It starts off like the Narnia series, where a young (tween) sister and brother are sent to live with relatives in a country estate.
If I recall correctly, the girl was older, and the boy was more of a child, she was closer to being a teen (or actually being a teen). I remember specifically that he was referred to as having baby fat still, and being more childish than her.
They discover salmon swimming in the pool, and there was an old woman. Turns out she was a druid who had turned a man into a salmon for (reasons). Both he and her are from the ancient past (you know, when druids were more common), and the guy starts to resemble the famous Irish hero Finn MacCool. I am pretty sure he was called Finn also, but I don't think they ever gave him a last name in the book, so I'm pretty sure it's not the eponymous book "Finn MacCool" which I saw later on.
The druid takes the kids and young man back in time with her, and they live in the woods for a while having adventures. I don't recall all of them, but I do recall that Finn had to catch and eat the "salmon of wisdom" or similar, and burned his thumb in doing so, which made him think he had to stick his thumb in his mouth to think.
Eventually the kids were returned to their own time, leaner and harder and more tan, and the boy faced up to some bullies and didn't let himself get pushed around (evidence of his character development, I suppose).
I thought it'd be a fun book for my daughter, but can't for the life of me remember the name to find it.