The Republic's access to UK fishing waters could be suspended to apply pressure on the country to fix a "constitutional defect", the UK fisheries minister said.
The Voisinage Arrangement between the two countries allows for mutual access to Northern Ireland and Republic of Ireland vessels up to six nautical miles off the coast of each country.
The agreement has been suspended in Ireland since the Supreme Court ruled in 2016 that it had not been incorporated properly into Irish law.
The UK has not suspended the operation of the agreement on its side and said it was committed to the principle of the deal and to working with Ireland to reinstate the arrangement.
Fisheries minister George Eustice said: "We have got a crucial period over the next few months as we finalise the (EU) withdrawal agreement and discuss a future economic partnership.
"Once we have fixed that, if the Irish Republic by that time have not reciprocated and fixed the constitutional deficit that they have, then we would have to consider at that point suspending their access, suspending the agreement so that there is some pressure on them to act."
He gave evidence to the Northern Ireland Affairs Committee of MPs yesterday about the impact of Brexit on fisheries.
Belfast-born Kate Hoey, a pro-Brexit Labour MP, questioned why the UK had not reciprocated what she termed the tough Brexit stance adopted by the Republic.
"Why are we so weak that we just accept that? In these negotiations while the Irish Government seem to be playing a hard ball way," she asked.