EU sources have downplayed prospects of a major breakthrough in the Brexit negotiations in the coming days.
It is understood there remains a significant gap between the EU and UK on the customs issue.
"There's no breakthrough yet. There have been some political commitments, but they haven't translated into the shape of a deal yet," one senior EU source told RTÉ News.
Michel Barnier, the EU's chief negotiator, briefed ambassadors from the 27 member states this afternoon on the state of play following two days of more intensive negotiations.
According to a European Commission spokesman, Mr Barnier told the ambassadors that while there had been constructive talks there was still "a long way to go".
RTÉ News understands the EU side continues to have concerns about the UK's proposal to have Northern Ireland be part of the UK's customs territory, but continuing to apply the EU's rules and procedures on customs and tariffs.
Sources say this remains "conceptually difficult".
It is felt that the British plan would create more problems than solutions, in terms of the potential for fraud, the difficulty of tracing goods and the prospect that things would not be ready in time for the end of the transition period.
Under the plan, which revives elements of Theresa May's New Customs Partnership idea from June 2018, Northern Ireland would remain in the UK customs territory but would apply the EU's tariff regime.
Goods destined for Northern Ireland would attract the EU tariff, but if that tariff was higher than the UK tariff then the importer would get a rebate.
If the goods were destined to cross the border south, then the EU tariff would be the correct one.
However, EU negotiators believe the system is fraught with legal and practical problems, such as how the authorities would be sure that goods officially destined for Northern Ireland actually remained there, and how the rebate system would work.
Officials have also expressed reservations over the use of goods that form ingredients that may end up in products exported over the border later.
They used the example of sugar. It may be exported from Great Britain to Northern Ireland but if the sugar is then used to make fizzy drinks which are traded south across the border, then there is a problem under what are called rules-of-origin.
One source said the chance of a deal at next week's EU summit were "very difficult, but not impossible".
Meanwhile, Minister for European Affairs Helen McEntee has said negotiations on a potential Brexit deal have intensified over the weekend and she hopes for a positive outcome, but stressed that the EU task force must be given time and space to do its work.
Speaking on RTÉ's The Week in Politics, Ms McEntee said that any progress on a way forward between the UK and the EU on Brexit could be presented to ambassadors to the EU tonight or tomorrow morning, but she urged caution at this delicate stage of talks.
She said that any potential deal will still need to be agreed by a majority in the House of Commons.
Ms McEntee said the Government does not know yet the detail of what is being discussed and any proposals must evolve and be developed to meet agreement with all the EU ambassadors.
She said that concerns around customs and consent remain, and added that Government departments have been preparing in 'crisis' mode for some time now.
Speaking on the same programme, Lisa Chambers, the Fianna Fáil spokesperson on Brexit, said there is still a long way to go.
She said we need to know what will happen if British Prime Minister Boris Johnson agrees a deal with the EU but cannot get agreement in Westminster to support it.
Sinn Féin's David Cullinane told the programme that the most important solution is one that sees Northern Ireland staying fully in the customs union and the single market.
Minister for Agriculture Michael Creed has said the Government trusts Britain in its quest to reach an agreement to avoid no deal.
Mr Creed has said while no formal Cabinet briefing has taken place, there has been informal contact between Cabinet colleagues about Taoiseach Leo Varadkar's meeting with Boris Johnson and "the citing of a pathway to a possible deal".
Speaking on RTÉ's This Week programme, Mr Creed said that getting any agreed, revised withdrawal treaty through the House of Commons in the tight timeframe poses many questions that cannot yet be answered.
He said no single party in Northern Ireland has a veto on what can be the outcome of these negotiations, and a way must be found that meets the concerns of all to ensure we do not revisit a chapter in history that no one wants to reopen.