Cauldron — A.J. Matthews

Actress Kate Susadi and Harvard academic Matt O'Brien are summoned to the residence of the enigmatic Collector, lured by a promise of wealth and adventure. Yet when the pair meet it's loathe at first sight. Matt thinks Kate's a beautiful ditz with some pretense at smarts: She thinks he's a patronizing WASPish stuffed shirt (although the shirt is stuffed with a mighty fine body...)
The Collector sends the pair to Galway, Ireland on a mission to recover a legendary artifact and find his old friend Thomas, Kate's grandfather, missing -- presumed dead -- for thirty years.
On the Galway shore they locate an ancient burial mound which holds the key to finding both man and relic -- but when they enter it they get more than they bargain for. Powerful sex magic plunges them through a strange portal into the legendary world of Eirin, a lethal and unforgiving place where gods and heroes walk the land and magic abounds. Somewhere here lies the Cauldron of Fire, the relic they must find to get them home -- but there're plenty of people who have a vested interest in keeping it just where it is. Kate and Matt's own squabbling and bickering doesn't help their cause -- until they begin to see the real person within the other.