Here's another two murder mystery authors easy to mix up (and both worth reading!).
Ellis Peters writes the Cadfeael series, which was also made into a TV series with Derek Jacobi, about a mediveal Welsh monk who solves various crimes.
Peter Beresford Elllis, a Celtic historian of some renown, using the name Peter Tremayne, writes the Sister Fidelma mysteries, about a mediveal Irish nun who solves various crimes.
If you like your Celtic history, you'll love these books!