Yes, yes - this is also a problem!
When is a tractor a tractor?
Although your link does not show it, the EU categories T1- T5 even specify the tyre width in determining which category the tractor falls under ...!
I think this is also a problem with rebuilt vehicles. if you go to the NCT to present the vehicle for registration and you state it is rebuilt, they will put you through the NSAI route (which undoubtedly costs money with no guarantee of passing).
If you say it is not rebuilt, then they ask you why was the car unregistered.
At this stage you have probably invested a lot of time restoring the car, it is capable of mechanical propulsion and is in good to excellent condition, sitting in front of the NCT inspector.
Remember you prob have no paperwork for the rebuilt vehicle having gathered up a shell and bits and pieces from everywhere. even if the shell was at one time registered that paperwork is prob missing. On older cars they only had the chassis no on a rivited plate and that might be gone too.
Try and argue that it was in a barn for 30 years ....
Note that the software the nct use automatically calculates the penalty based on the date they enter into it when you are there. That was the prob with the guy with the fergy in the FJ. he signed the declaration there. Toough luck acc to revenue.
Undoubtedly, this is where the search for a matching log book comes in - thus the registration (and nsai verification) process is completely avoided/evaded.
If the car is pre 81 , it will not do an nct test so no one will check the chassis no on the log book against that on the car ( if it is on it).
The chances of a Garda actually doing this or having recourse to do this approach zero rapidly too ...
see what I mean?