Senator Jerry Buttimer has resigned as Leas-Cathaoirleach of the Seanad.
He is the latest resignation in the wake of an Oireachtas Golf Society event in Galway on Wednesday night.
The event saw some 80 people attend, which contravenes public health guidelines that only 50 people should gather indoors.
Gardaí are now investigating the event for alleged breaches of public health legislation.
Mr Buttimer also attended the event, which he said this was a "serious lapse of judgement".
He said: "My attendance at a dinner event in Galway this week has compromised the Government at a time when people, across every sector of Irish society, are doing their best to keep all safe during this global pandemic.
"It was an unintended but serious lapse of judgement in attending the event.
"I should not have attended the dinner and hereby tender my resignation as Leas-Cathaoirleach of Seanad Éireann to the Cathaoirleach of the 26th Seanad.
"I apologise unreservedly for my actions this week.
"For the last six months, the country has come and worked together, to defeat COVID-19.
"It is an ongoing battle and our collective societal response is important and our individual actions matter.
"I wish everyone every success in this fight", he added.
My letter to Cathaoirleach of the Seanad @SenatorMarkDaly pic.twitter.com/m0hCqyZJC8
— Senator Jerry Buttimer (@jerrybuttimer) August 21, 2020