story identification - 80s book similar to Narnia but with a more Celt feel, allusions to Finn MacCool and eating "the salmon of wisdom"

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.

Answer

This sounds like The Wizard Children of Finn, by Mary Tannen. It was published in 1981, so the time frame is right. It has the same basic plot: girl and her brother go back in time to the age of Finn. The boy is not named Finn, though, but Bran, though the legendary Finn does show up in the story.

The ages match your recollection: the girl is older, and the boy is younger:

Fiona McCool, age eleven, was kneeling on the worn silk-covered loveseat on the front stair landing, which was the best place in the house to watch anyone coming or going. Not only could you get a perfect view, but Fiona had noted that if you stayed on the loveseat, which was two feet away from the window, you couldn’t be seen by anyone standing in the driveway below.

Beside Fiona was Bran McCool, age eight, Fiona’s brother and for the moment her prisoner, for she was holding him tightly by the wrist, and if he tried to move, she gouged her fingernails into his flesh.

The bit about baby fat:

Bran stood before the fire. With everyone sitting down, he looked much taller than he really was. Hiking through the woods for a couple of weeks had gotten rid of the baby fat. His face was harder, more boy like.

There's a druid who turns someone into a salmon:

“I think,” said Bran, “the first thing we gotta do is find that kid Bovmall turned into a fish.” “

Yeah,” said Fiona. “At least he will believe us. That’s one more on our side. Jeezle, Bran, why didn’t you think of it before? That boy might be salmon croquettes by now! ”

And so forth.