May agreed in December that the fall back position, if there is no trade agreement, the north of the island of Ireland will stay in the customs union and the single market. In other words no border on the island of Ireland. This was a major u turn from what she said in 2016, which was that there will have to be border checks on goods and people on the island of Ireland.
As far as the Irish were concerned they took May at her word in December. But to their astonishment May wants to renege on what was agreed in December and she is now back to her 2016 position that there will have to be border checks. The Irish border issue is firmly in the UK's court, as is the border at Dover, or Liverpool, or Birmingham etc.. The UK are now saying they will not abide by what was agreed in December. It is extremely difficult for the RoI, or the EU to help the UK government resolve its border issue if it can't stick to an agreement. What the UK government agreed to in December will resolve the border issue. If May now chooses to go back to her position in 2016, and therefore not abide by what was agreed in December, then the UK will inevitably put border checks in place.