OK lets break this one down

Wilmol wrote: »

Have a friend in NI who has a mauser 98k but it's claimed length is around 23.6.

Ok...no idea as it is NI and they have separate legislation to the ROI and UK on firearms.So what their minimum barrel length is for rifles...Someone here might answer. You are also talking about two different types of guns that have different legislation applying to them down here and in NI

Where is the restriction specified?

Firearms act 1925 or 1928?.Lays it out minimum length for a shotgun barrel

Seems stupid to have this arbitrary 24inch min length, what is the reason for it if actually is a valid requirement?

Seems to be a hangover from the UK times here.UK legislation is 24 inches minimum length too. Seems to be a compromise between shortness and not easily concealed under a coat? Very much a valid under law requirement.

So if I want a Remington 870, it has to have that stupidly long barrel?

If you want it unrestricted yes. If you want it shorter,get a restricted license, and buy a shorter barrel that brings the gun down to 60 cms overall length under EU legislation,and adapted here too. And have a good reason as to why you want such a short shotgun to your chief super?
Unless you are planning a career in post office heists there isn't much you can do with that short a barrel here, and unless you get it choked you'll just be spreading shot all over the place at a short distance.