So, am I right that Michael Smiley as Colonel Blue is only the third character ever in Doctor Who with an Ulster accent?
The other two that I can think of are Harry Towb as McDermott in Terror of the Autons (1971) and Declan Mulholland as Clark in The Sea Devils (1972). Both appeared in other Who stories with non-Ulster accents.
(The daughter of the 7th Viscount Bangor was pretty visible in Who at one stage, but she sounds pretty English these days.)
Edited to add: I had forgotten Jonjo O’Neill as McGillop in Day of the Doctor (2013).