Micheál Martin said the UK was “under no illusions” about the strength of opposition to its unilateral actions but he was “hopeful” the talks could get to a more “intense phase over the next week or so”.
He made the comments following a bilateral meeting with EU Commission President Ursula von der Leyen in Brussels.
The Taoiseach travelled to Belgium to attend a special meeting of the European Council on Thursday.
It comes as the EU announced it will begin legal proceedings over Boris Johnson’s Internal Market Bill, which overrides key elements of the Brexit Withdrawal Agreement relating to Northern Ireland.
Mr Martin said: “We support the Commission in its actions.
“It gave the United Kingdom a month to respond in relation to withdrawing the Internal Market Bill and the unilateral action it took in relation to the Northern Ireland protocol.
“I met with the president, she appraised me of the decision taken by the Commission.
“We also discussed how we will deal with Brexit over the coming weeks.”
The leader of Fianna Fail told RTE News there was “concern” across Europe at the unilateral action by the British government on the Northern Ireland protocol and that the UK Government needed to “resile” from that.
“Parallel with that we are conscious of the need to conclude a comprehensive relationship agreement with the UK which is in the best interests of the people of Europe, the UK and Ireland,” he said.
“A sensible free trade agreement is something that we are seeking also.”
While Mr Martin said he was hopeful that talks would become more intense he also cautioned that the issues were “very substantive”.
“I think the mood is moving in the right direction towards better engagement, that’s always important in terms of negotiations of this kind but there are serious challenges ahead.”
He said it would “remain to be seen” how the UK would respond to the EU’s actions.
“The UK Government is very conscious of the strength of opposition to its unilateral action across Europe and indeed in Ireland,” Mr Martin said.
“It is under no illusions about that.
“It understands that.”
He added that Ireland was willing to play its part together with the EU in advancing the talks.
“I have said repeatedly and consistently that it is in the interests of the people that we represent, their livelihoods, their jobs, that all political leaders work collectively to make sure we get the best outcome for workers, for businesses and the people generally,” he said.
“That is ultimately our objective as a country.”
Mr Martin has previously said the Irish Government was preparing its latest budget on the basis of a no-deal Brexit.
He recently told the UK Liberal Democrats’ conference that he was “not that optimistic” of a future free trade agreement being reached between the UK and the EU.
While in Brussels Mr Martin will attend the Renew Europe working lunch with other EU leaders ahead of the Council summit.
Participants will discuss how to deepen and strengthen the Single Market as a key tool to drive Europe’s economic recovery.
It will also consider how to improve the competitiveness of European industry.