I have no axe to grind on this as you are all modelling the wrong scale anyway
No point messing with 20.2mm gauge or whatever, if 21mm doesn't suit then save yourself a load of time and effort and stick with 16.5mm.
There is no snobbery amongst the modellers of the Irish scene and nobody will look down their noses at you for using the "wrong" gauge track.
Generally, if you are going to model any minority scale/gauge combination then you are going to have to get your hands dirty and do some scratchbuilding along the way, for the 4mm/21mm gauge that means track and especially pointwork.
For the few parts of the system that used Bullhead then the existing C&L (or similar) injection moulded chairs and sleepers can be used, fairly straight forward with a bit of practice, pre machined crossings and blades are readily available for those that need them and Templot can generate the templates (with a bit of practice or a helpful friend). The pain of doing yards of plain track can be eased with the latest developments in Templot plug/COT track and a home 3d printer (or another friend who has one?).
Flatbottom plain track is feasable with a home FDM 3d printer as my experiments with the S Scale code 75 are showing and no doubt Paul will document his progress on Broombridge in the fullness of time. Even the cheapest of the FDM printers will do it and the material cost (sleepers, not rail!) is just pennies.
Of course back in the "old days" the likes of Mike Sharman, Colin Binnie and Roy Link would have just milled up a simple brass die and squirted hot plastic around the kitchen with their homemade injection moulding machines - in fact Roy did with his 7mm/14mm gauge NG track although that needs the rails spiking down through the pre-moulded holes.
If someone was going to put their hands in their pockets for a few sleepers worth of injection moulded plain track base is there a consesus on what rail size to use?
Code 75, 83 or 100?
Enough of my rambling..
Rob