The wider reopening of pubs from coronavirus restrictions later in the month is at risk over concerns about social distancing outside bars.
Taoiseach Micheál Martin said the Government was unhappy about incidents at some locations over the weekend and warned plans for the next phase of reopening remained "under review".
Separately, holiday-makers will have to wait to find out which countries will open for non-essential international travel after the Cabinet put off publishing a so-called 'green list' until July 20.
Pubs that sell food were allowed to reopen last week but the remainder of licensed premises can't reopen until phase four of the Government's 'roadmap' due to begin on the same date, July 20.
Major concern has been sparked by videos that emerged on social media over the weekend showing hundreds of young people congregating to drink on Dame Lane in Dublin city centre.
Mr Martin said the majority of pubs that had already opened were compliant with the rules, including limits on how long customers could stay in the premises.
But he also said it was a "challenging weekend" and there had been "difficulties" in social distancing where alcohol was sold for take-away purposes.
He said the regulations were being strictly applied by the industry but added: "We will be keeping it all under review in terms of phase four. "Nothing can be guaranteed if this behaviour were to continue."
Asked whether this meant it was possible the remaining pubs wouldn't reopen as planned, he said: "We want pubs to reopen safely on the July 20 but we can't be blind to what's going on right now - those incidents where the guidance wasn't adhered to.
"We are keeping it under review. We're not happy with what transpired in certain locations over the weekend."
He said a new contact-tracing app to be launched today was a welcome development as part of the coronavirus testing and tracing regime.
Health Minister Stephen Donnelly said more than 80pc of people were willing to download the app, which will help trace contacts of coronavirus patients who aren't known to the individual.
He referred to the "scenes we saw over the weekend" and said: "If one of those people tested positive, there might be five, 10, 15 people they are in close contact with and they have no idea who they are. Everyone who has the app would get a notification that they have been deemed a close contact."
Mr Donnelly also said public health officials were "very worried about the potential for a second surge really driven by a big upswing in foreign travel".
He said that 17pc of new coronavirus cases here were now caused by foreign travel.
Mr Martin said airlines promoting tourist flights now were acting "contrary" to public health advice. The former caretaker government had suggested a 'green list' of countries where travel would be allowed, without the need to quarantine for 14 days on return, would be drawn up this week. Mr Martin confirmed the list now wouldn't be published until July 20.
He said the Government didn't want to undermine the sacrifices made by the Irish people and that was why a "cautious approach" was being adopted for international travel. He pointed to new spikes of coronavirus cases in Spain and the UK and said: "That is informing our view as second waves are emerging across Europe."
He said Ireland wanted to keep the number of cases low to allow for the full reopening of schools and non-Covid-19 health care services. The green list will be of countries that have similar numbers of virus cases to Ireland.
Ireland, which has recorded no new deaths from Covid-19 for two days, could run into trouble again if the weekend scenes of heaving revellers enjoying "takeaway pints" in Dublin city are repeated, public health doctors warned.