Laoghaire is a pretty girl who lives at Castle Leoch. She has the hots for Jamie and is subject to Claire's jealousy, though Claire has no reason to be jealous since Jamie has no interest in the girl, and barely acknowledges her once he's married to Claire. Claire is, however, right to be wary of Laoghaire since she almost kills Claire when she lures her to Geillis Duncan's house on the day of the witch hunt.
Laoghaire serves as a foil to Claire. Claire is mature, knowledgeable, and levelheaded, all things with Laoghaire decidedly is not. As Alec the stable master says at one point, Claire is a woman and Laoghaire is a girl. And Jamie Fraser deserves a woman, and not a girl. But girls get jealous, and Laoghaire lashes out in just this way, almost killing Claire in the process. Yeah, we can't imagine her and Jamie ever getting along. Good thing he'll never have to see her again… right?