Irish Agriculture Minister Dara Calleary has resigned after attending a golf dinner with more than 80 people.
Wednesday's Oireachtas Golf Society event at a County Galway hotel came a day after Dublin announced a tightening of lockdown restrictions.
Dara Calleary had been in post for a month; he replaced Barry Cowen who was sacked after a drink-driving scandal.
The Irish PM accepted the resignation, saying Mr Calleary's behaviour "was wrong and an error of judgement".
In a statement on Friday, Taioseach Micheál Martin said: "People all over the country have made very difficult, personal sacrifices in their family lives and in their businesses to comply with Covid regulations."
"This event should not have gone ahead in the manner it did, given the government decision of last Tuesday."
Gardaí (Irish police) are now investigating the event for possible breaches of Covid-19 regulations.
EU commissioner, Supreme Court judge, senator...
Mr Calleary apologised profusely in a series of tweets on Thursday, saying: "In light of the updated public health guidance this week I should not have attended the event."
"I wish to apologise unreservedly to the public, from whom we are asking quite a lot at this difficult time. I also offer this apology and my sincere regret to my government colleagues."
Others present at the event included the former Fine Gael minister and EU Commissioner, Phil Hogan, the Supreme Court judge Séamus Woulfe, Fine Gael senator Jerry Buttimer and the Independent TD (MP) Noel Grealish.
Mr Buttimer, the Leas-Chathaoirleach (deputy chairman) of the Seanad (Senate and upper House) has now also resigned.
Analysis: Shane Harrison, BBC News NI Dublin correspondent
Dara Calleary has now joined Michelle O'Neill and Dominic Cummings as a senior person in political life in the UK and Ireland caught-up in a controversy over whether they broke their own Covid-19 rules.
But he is unique in resigning.
And so the new Taoiseach Mícheál Martin is now looking for his third Agriculture minister in less than two months.
It has been a bad start for the new coalition government with not even the summer recess providing a break from political mishap.
Once again the issue - as was the case in Northern Ireland and England - was whether senior political figures broke their own administration's rules giving rise to the impression that there is one rule for ordinary people and another for the political elite.
But it's not just the Fianna Fáil part of the coalition government that has been caught up in this affair.
Pressure is now building on some associated with Fine Gael including the EU Commissioner Phil Hogan and Séamus Woulfe, a supreme court judge and former attorney general in a Fine Gael-led government.
The Leas-Chathaoirleach (deputy chairman) of the Seanad (Senate and upper House) Senator Jerry Buttimer who was also there has now resigned.
Dara Calleary is not to be the only casualty of Wednesday's Irish parliamentary golf dinner
Fine Gael Senator Jerry Buttimer has also apologised for attending the event saying he was "mindful of the tremendous effort made by everyone since March to try and beat the virus".
Updated guidelines
In tightening restrictions in the Republic of Ireland, the number of people able to attend indoor gatherings was limited to six, with some exceptions.
Previously, indoor gatherings with up to 50 people had been permitted.
The 81 people at the County Galway event were split across two rooms.
James Sweeney, from the Station House Hotel where the event was held, told RTÉ he had checked with the Irish Hotels Federation to ensure the event was compliant.
He said he was told it would be if the guests were in two separate rooms, with fewer than 50 people in each.
Mr Calleary replaced Barry Cowen as agriculture minister last month after his predecessor was sacked over controversy around a drink-driving ban.
Micheál Martin will now have to appoint his third Agriculture minister since the end of June when his coalition government was elected.
The Republic's cabinet is due to meet later on Friday to discuss easing the local lockdowns in counties Laois, Offaly and Kildare.