The Pillars of the World (Tir Alainn Trilogy)

The youngest in a long line of witches, Ari senses things are changing for the worse. For generations, her kin have tended the Old Places, keeping the land safe and fertile. Now, she finds herself torn between the world of mortals and the world of Fae, who ignore what occurs in the mortal world, for the roads between the two lands are vanishing into thin air.