Contexts.
8.7% of Irish Trade is with GB.
It accounts for 2% of GDP.
BUT ! It is high volume/low value. It is agrifood. It is employment of hundreds of thousands of real people.
Let me make this simple

A small trader butcher in Liverpool is delivered 100Kg of FRESH meat daily from Dublin. He has 3 shops. He sells it that day.
He can't buy a container 40 tonnes of frozen crap from USA. He can't store it, his customers don't want it. He can't afford it.

In January 2021, he has extra costs. (10% in paperwork, vet certs ). If GB sets a 0% tariff on imported beef, it means nothing to him or his customers. It means, technically he can buy a million tonnes and save money .

His customers don't want the product. He can't afford the massive costs of storage nor the export credit insurance that the supplier demands.
Customers want what they trust and are used to.
That is how business works.

wibble wibble https://www.thejournal.ie/amount-ireland-trades-with-the-uk-4745788-Jul2019/

So, put the " deal with USA " aside.
My butcher with 3 shops will still buy what his customers want. It will cost him/her and the Irish supplier. They have a 50 year relationship and will share the costs . They will help eachother survives as best they can.

A personal note. I was a chef. I was taught in France and ireland, have my whites,my medals, have owned two restos. I have also eaten steaks in the oldest resto in USA (if it's true, New Hamphire" ( $385). I have eaten steaks in Texas (size of a car)... all cardboard.