"Nan's Ghost" is most unusual, as it's a two-part episode...something I don't think they'd ever done before. Because of this, it felt a bit overlong and drug out, but not obviously so and the overall story is quite intriguing.
Episode one finds Jessica once again in Ireland and she's looking into the supposed accidental death of a girl she knew in a castle which MIGHT harbor treasure from the 17th century. Naturally, while the police just chalk the death up to an accident, Jessica's murder senses tell her there's far more to it. This episode ends with Jessica being locked in a hidden portion of the castle...and no one can hear her or knows where she's gotten to.
The story is fresh and interesting and features a nice setting...even though it really wasn't filmed in Ireland. The second unit who filmed various sites in Ireland provided some nice color.
You might wonder why this show so often has Irish episodes. I think it could easily be because Angela Lansbury actually lived there for some time and must have loved the country.