Referring back to the OP. There WILL be a border down the Irish Sea, passports and goods will be checked, having said that there will be agreements covering many products, particularly agricultural. It is in Ireland’s interest to get agreement quickly, their beef and dairy industries are highly geared to the UK.
As for food imports from other countries, the UK has said loud and clear there will be no relaxation on our welfare or environmental standards. Fisheries are uncertain, the UK will regain control of our waters and allocate permits to fish. Don’t expect many more UK boats, many are crewed now by migrants, the work is too hard for UK workers.
Exports to the EU will be under pressure, there will be losers.
Success or failure will be judged at the next election when most deals will have been done