This was book written in either the 1940s or 50s. set in Ireland. A girl goes to visit family in Ireland from England. While there, she goes to a mysterious ritual on an island. There is a chariot race that she didn’t realize would entail her being engaged.
Oh wow. I read that book too, but didn’t remember it until I read this.
I think the author was Mc or Mac-something. McMoy? MacNamara? And I think the title has green grass or grass island in it.
Not very helpful, but maybe it will jog someone else’s memory.
This is the book I came looking to find! From my recollection, she takes a small round boat (coracle) and it drifts to the island, there’s an entire society hidden on the island, and that she has to race her friend (Milo, I think). She wins and is engaged to him (the prince?) but manages to get away from the island.