A Wizard Abroad, by Diane Duane
In the 1980s, Diane Duane moved to Ireland. This novel is clearly her love letter to her adopted homeland.
Nita's parents worry about how much time she's spending on her wizardry, so they send her to her aunt's farm in Ireland. While there she, of course, finds wizardly work to do. And that's before it becomes evident that Ireland is in crisis, and that she's not really on vacation, but is on wizardly loan to Ireland.
Kit and Dairine have roles to play in this as well, and we meet another new young wizard who will have roles to play in later books, as well.
The only negative to the book is the fairies. Not little pixie, Tinkerbell-type fairies, but human-sized ones. And I'm sure that, as fairies go, they're just fine. But I'm just not a fairy-type person, I guess.* That whole thing leaves me feeling "meh."
Fortunately, the fairies are a small, small part of it, so I can otherwise wholeheartedly recommend this delightful book.
*I tried reading "This Scepter'd Isle" once. Once. My ex-husband tried to get me into the Artemis Fowl books, too. "Meh" just about says it.