A Wizard Abroad (by Diane Duane)

When Nita's parents decide she needs a break from wizardry, they ship her off to stay with her Aunt Annie on a farm outside Dublin. Ghost horses and a talking cat named Tualha are the first clues that magic surrounds her. Researching Irish mythology at the library, Nita learns that 400 million years ago, Ireland was geologically severed. .

When the Makers tried to reshape the land, the powerful Fomori interfered. As a result, it's too easy to accidentally slip sideways through time and space. Nita the wizard must do something about the leakage. With the help of her new love interest, Ronan, she learns of the Four Treasures, three of which are still in existence--the cup, the sword, and the stone. But a fourth, the spear, must be reforged, to enter into a battle with the Fomori and Balor, the new form the Lone Power has taken.

Fantasy readers will find they are on familiar ground with this tale of a quest for the Fourth Treasure, and the awakening of the first three objects. But while Dairine has a definite role to play, it's never quite clear why Nita and Kit were called all the way to Ireland for this mission. The story rises to a fevered pitch that culminates in a strange battle scene. "... the drow exploded into fragments and splinters of stone that bled hot and splattered her with ichor that burned like drops of lava. Her wizard's shield took most of it, but a few drops got through ... and burned right through her clothes to her skin."