Neasa Hourigan says the ban on fossil fuels for cars for 2030 will be implemented, despite not being included in the government's Climate Action Bill.
The ban was included in the national development plan and the programme for government, and many had expected the bill to include the issue.
The Bill, approved at cabinet this week, sets a target to de-carbonise the economy by 2050 at the latest.
Ms Hourigan, Green TD said the government decided that because the issue is within EU law, it might "actually hold up the climate bill" which is domestic policy.
She said her party has "learned this the hard way" during the "microbeads issue" when it had attempted to ban the microplastics in facewashes and shower gels.
"EU competition law kicks in because we would have to notify them, not just of the law itself, but of the proposal for the law.
"Now we're very much hoping to implement this law by the beginning of December. We cannot individually change our laws on petrol and diesel cars here without formally notifying the EU there is a change to the parameters of trade.
"We've discussed it within the Brexit negotiations about that level playing field, about competition law. Now, we could include it in this bill, but we'd be very likely to frustrate the bill and hold it up.
"It's still going to happen, but we decided not to include it in this particular bill because we're trying to do it on a quite tight timeframe.
"We fully intend to bring it in."
When asked if voters had been let down, Ms Hourigan said: "I think everybody knows that I'm well able to criticise the government where I think it's warranted but I think in this case it's actually a wiser choice. "
Ms Hourigan did concede on Newstalk radio that "a whole load of issues" aren't getting the focus that they need to because the pandemic is overtaking all other issues, and when asked about Tánaiste Leo Varadkar's comments on Nphet on Sunday said she is "not a fan of criticising" the health body.
"I wouldn't have approached it the way he approached it."