Most high tension lines here (110kV, 220kV etc) will "sizzle" during very damp conditions.
Irish drizzle can produce some unusually difficult conditions for electrical installations. You basically get a dense fine mist of rain that can cause all of the above effects.
Also, some of those 110kV lines in N.I. are probabally from the same era (they look simlar) to the very oldest parts of the national grid in the Republic. It's very unusual to see a corona, even on the oldest bits of the network though.
