I've spoken to a couple of Roads Service planners about the imposition (or lack) of motorway restrictions on new road schemes, eg the upgraded Westlink. They seem to take the view that if it makes no different to road users, then there is no point in doing it. When I asked about Westlink the response was "What difference would it make? The road would be no different."
The ostensible reason for no motorway restrictions on the A1 is the lack of an alternative route, but I've come to think that that wouldn't really be an obstacle if they really wanted to do it. The only stretch without a convenient alternative route is this short stretch
. However, given there are a number of local roads nearby, surely something appropriate for banned traffic could be cobbled together. The stretch of the Newry Bypass north of that (as far as the A25) was previously the all-purpose S2 Newry Bypass. However, it was only built in the 1990s, so is hardly likely to raise a legitimate "right of way" objection.
One point to consider is that "motorways" are a dirty word in the UK, in a way that it's not (yet) in the Republic. Proposing a new "motorway" is likely to garner more opposition than an identical all-purpose road.
So I think the real reason for no motorway restrictions is basically that Roads Service (a) don't see the point and (b) don't want to create needless hassle for themselves.