I could not find the Northern Ireland equivalent of Great Britain's "clipped and generalised" shapefiles one can download from the ONS. Almost half of the final GeoJSON file is due only to the extreme detail of that.
b041bee358fcf55d7e8bf187e0543427c01517f0 reduces the size of the GeoJSON substantially, but it is still likely too big, need to experiment using _STSimplifyPreserveTopology.
I could not find the Northern Ireland equivalent of Great Britain's "clipped and generalised" shapefiles one can download from the ONS. Almost half of the final GeoJSON file is due only to the extreme detail of that.