As I understand it, the hard option involves the UK applying tariffs at zero - to all WTO members - except for dairy and beef, where some tariff quota arrangements favouring Ireland would apply, which would pretty much maintain the status quo on Irish exports to the UK.
I don’t know how much trade goes on between Ireland and Northern Ireland in those products compared to the mainland. But it could be managed by a certificate system, obviating the need for border checks, and where smuggling could be contained
For all other products, it would be as you were for imports from Ireland.
Under this scenario, UK exports to Ireland would be subject to the EU tariff. Exports to Ireland via Northern Ireland could also be managed by some sort of pre paid tariff certification delivered at the point of destination. As it is, some customs clearance is not done at thenational border but at designated customs clearance points closer to the point of consumption. Smuggling is manageable.
I doubt that the UK would export much agriculture to Ireland and EU tariffs on other goods are low to minimal.