How would they impose tariffs as required by the WTO?
The WTO doesn't require the UK to impose (or more pertinently enforce) tarrifs on Irish imports.
It just requires that imports from all other members are treated equally.
That means we have to let stuff from, for example, China, Columbia, Congo or Afghanistan through with a remote declaration and no check at the border.
So we can do all that stuff at the Irish border so long as we don't have any other real borders. No probs.