Dark Witch by Nora Roberts

Dark Witch (Deckle Edge) (The Cousins O'Dwyer Trilogy) - In 1263, Sorcha, the Dark Witch, tangles with Cabhan. Cabhan wants Sorcha's powers.  Sorcha has three children to whom she entrusted her magick just before she burned Cabhan and died herself.  In 2013, Iona Sheehan moves to Ireland from America at the urging of her grandmother.  In County Mayo, she meets her cousins Branna and Connor O'Dwyer.  Branna recommends she work for Boyle McGrath and Finbar Burke who owned the stables and ran guided tours.

Branna, Connor, Fin, Boyle, and Meara (who works for Boyle and Fin) grew up together knowing that Branna, Connor, and Fin are witches.  Branna, Connor, and Iona descended from Sorcha. Fin descended from Cabhan.  Iona, however, is just learning about her powers, however, Cabhan knows that the three are together and begins to assault Iona, the weakest, to get her to turn her power over to him.  The four witches, with Meara and Boyle, ban together to fight Cabhan.