Senator Martin Conway has resigned from the Fine Gael parliamentary party as a disciplinary process gets under way following his arrest last month.
In a statement, the Clare-based senator said he was arrested on 22 January on O'Connell Street in Dublin "for being in an intoxicated state due to consumption of sleeping tablets and alcohol".
He said: "I was detained at Store Street Garda Station and subsequently released without charge a number of hours later.
"This was not disclosed to the party."
Mr Conway added: "I would like to apologise unreservedly for this to my family, colleagues, supporters and my nominating body, Vision Ireland. I let myself down.
"I would also like to apologise to the members of An Garda Síochána for having to deal with this matter.
"I have resigned from the Fine Gael Parliamentary Party.
"I understand that a disciplinary process will now commence."
Fine Gael said in a separate statement that it noted Mr Conway's resignation and apology.
The party said it is now subject to a party disciplinary process.
Mr Conway was elected as a senator to the Administrative Panel last week and has served as a member of the Seanad for 14 years.
In 2011, he became the first visually impaired Oireachtas member when he was first elected to Seanad Éireann.
Minister for Agriculture Martin Heydon welcomed what he described as Mr Conway's "swift statement and action".
Speaking on RTÉ's The Week in Politics, Mr Heydon said he was very disappointed with the situation but that ultimately Fine Gael politicians are held to a high standard as public representatives.
"We sign a party pledge committing to upholding these standards and informing the party of any instances like these," he added.
The party wasn't informed in this instance, the minister said, and a disciplinary process is now under way.
