The Last Killiney, the first book in The Ravenna Evans Series.

On holiday in England, Ravenna Evans begins to recall her past life as an eighteenth-century Irish viscount’s lover. When she is swept back in time to relive this affair (along with Lord Killiney’s very unwilling modern counterpart), she learns that living in the past isn’t all fun and games, and that history can’t always be changed to one’s liking.