There are two general heights for double deckers - 14'6" AND 13'6" - I would imagine that the UTA didn't have many low ones.
Thus 58 or 54mm in 4mm/ft.
On two out of the eight 141/181s that I've been associated with, we needed to shave a tiny bit off to ensure clearance when the plate was fitted, the other six were OK*.
* or might just not have had the plates fitted...
Smuggling migrants from Mullingar to Port Laoise.
I did see a discussion about it somewhere a few weeks ago, but I can't remember the outcome, hopefully somebody else was paying more attention.