The Fine Gael leader has said he expects the highest standards to be upheld in the party's disciplinary process in relation to actions of the former parliamentary party member, Martin Conway.
The former Senator resigned from the Fine Gael parliamentary party under way following his arrest on O'Connell Street in Dublin last month.
In a statement, the Clare-based senator said he was arrested on 22 January "for being in an intoxicated state due to consumption of sleeping tablets and alcohol".
Simon Harris said he is aware of "distressing circumstances" surrounding the matter and called for a thorough investigation.
"That is not a normal scenario for a parliamentarian to find themselves in and it is appropriate that he resigned the whip."
Speaking on his way into a meeting of the Cabinet, Mr Harris also acknowledged a "previous issue" dating backing to 2018, but said it would be "unusual" for a party leader to reopen a matter which had not required any further action.
It is understood that Mr Conway had been accused of acting in an inappropriate manner with a female Oireachtas staff member working for a Fine Gael politician in a bar in Dublin.
"There was a previous issue that predates my leadership by many years," Mr Harris said.
"It came to the party's attention in 2020. I became aware of this late last year."
"From my perspective, when the issue had been examined, an apology had been issued and it was deemed at that stage that no further action was required.
"It would be very unusual thing for a new leader to then reopen a matter that in that context was deemed to require no further action many years past.
"I don't want to over-comment on this until the disciplinary process has come to a conclusion.
"But let me say this, I expect the highest standards to be upheld."
