Senator Buttimer was one among over 80 people who attended an Oireachtas Golf Society event on Wednesday which appeared to breach public health guidelines.
Dara Calleary has also resigned as Minister for Agriculture following his attendance at the event after the Irish Examiner broke the story last night.
Mr Buttimer posted a letter of his resignation from the position on social media this morning.
He said his 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 I herby 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, this 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."
There is mounting pressure on him to resign from the Seanad following his attendance at the event.
Last night, Mr Buttimer confirmed his attendance at the event and apologised on social media.
He wrote on Twitter: "I attended an event at the Station House Hotel in Connemara last night [Tuesday].
"In light of the updated public health advice, I should not have attended. I apologise for this, in doing so I am mindful of the tremendous effort made by everyone since March to try & beat this virus, flattening the curve by adhering to public health advice & lockdown measures.
"This was a lapse of judgement on my part for which I sincerely apologise."
