You're quite right, minister - I had forgotten that. 00n3 or TT is indeed what's needed.
There was a lot of dual gauge track in Larne Harbour station at one time. After the last remaining bit of the Ballymena & Larne closed in 1950, fragments remained about the yard almost until its removal in the early 1960s.
At Strabane, one might have expected the same, but it was nit so, other than a wagon turntable. The Derry docks had a lot, and the RPSI's 0.6.0T "R H Smyth" had a narrow gauge chopper coupling set off centre on each buffer beam so that it could haul CDR or LLSR stick as well as NCC or GNR stock around the quays.