"What of the idea?"

I don't like it. It looks like an attempt to manoeuvre the whole of Ireland into being part of the EU. I prefer the view of the government spokeswoman which is shown in that same article (and quoted above):
She said: “Our policy is clear – we are committed to ensuring there is no hard border between Northern Ireland and Ireland and to ensuring the same unfettered access for Northern Ireland’s businesses to the whole of the UK’s internal market. We have set out our preferred customs models to enable trade to remain as frictionless as possible.”