Under the Protocol the UK government is responsible for applying checks on goods entering NI from Great Britain to ensure that goods destined for the Republic of Ireland meet EU regulations. This is to avoid a "hard border" between the Republic and NI - i.e. avoid any need for checks at the land border.
The UK government has made clear that, in accordance with the Good Friday/Belfast Agreement, it would never, under any circumstances, undertake any checks at the land border.
Article 16 of the Protocol states that "if the application of this Protocol leads to serious economic, societal or environmental difficulties that are liable to persist, or to diversion of trade, the Union or the United Kingdom may unilaterally take appropriate safeguard measures. Such safeguard measures shall be restricted with regard to their scope and duration to what is strictly necessary in order to remedy the situation. Priority shall be given to such measures as will least disturb the functioning of this Protocol."
The UK government believes that the Protocol has resulted in diversion of trade (and it is difficult to dispute this as a factual proposition) and so is entitled to take Art 16 safeguard measures. It has not yet done so but says it may have to soon.
If the UK did take Art 16 safeguard measures which resulted in less checks on goods entering NI from GB and that resulted in goods circulating in the Republic which had not been checked for compliance with EU regulations:
1.Would the EU tell the Republic of Ireland that it must make checks on goods crossing the land border from NI to the Republic?
2.If the answer to 1 is Yes, would the Republic do what the EU tells it to do (and so breach the Good Friday/Belfast Agreement) or would the Republic stand its ground and refuse to do what the EU tells them to do?
3.If the Republic did stand its ground, and refuse to implement a hard land border, what is the EU likely to do in response?