It's here in the bowels of the parliamentary complex where some of the most radical policies that could shape a coalition government in a generation have been hatched.

During an hour-long interview, Ryan mulls over the post Covid-19 pandemic recovery and what a triumvirate coalition involving the Greens, Fine Gael and Fianna Fail can and can't do.

The three party leaders are up against a looming deadline this week to fine-tune a deal, with outstanding disagreements over transport, agriculture and housing among issues.

Ryan also faces his own internal party struggles, with the announcement that deputy leader Catherine Martin wants his job.

Despite doomsday predictions from fiscal chiefs about the State's deficit running close to €30bn this year, Ryan talks about a new green dawn. Yes, there will be difficult choices, ones that could bring a government down. But the pandemic will allow green politics flourish, he firmly believes.

With a government deal between the three parties set to be agreed this week, he tells me of the final crunch talks, whether he could be Tanaiste and if he can bring the Green grassroots with him.

The looming recession from the pandemic doesn't compare to the 1980s or the most recent one in 2008, the Green leader maintains. During the former, borrowing was at double digit figures while the latter saw a structural deficit from the collapse in construction.

This time, job and industry numbers don't compare and so the challenge is breathing life back into them.

So the key economic here isn't a structural change, it is to get it [the economy] back working. And that is a stimulus, getting demand back and getting people back to work. That's the key thing.

"And in doing that, our tax revenues, not just our income tax, but Vat and so on, will come back into line, and it should be manageable.

“It'll probably take two or three years so - if all goes well - for that to happen. It depends on Brexit, it depends on a second wave of Covid or how big it is. But that is the assumption. And that's two or three years when you're borrowing extensively, when you're actually putting in stimulus measures to get the thing going again.”

Those early years are when the next government will borrow heavily, create jobs but also focus on capital projects. Expect stimulus plans to figure strongly in a recovery plan.

Ryan explains where those borrowed monies may go:

“There are structural issues still around housing, public transport, water infrastructure. And in fact, there's a whole range of climate investments we need to make in renewable energy, in new electric vehicles, new walking/cycling infrastructure, new better ways of doing farming. That is a structural change that's coming and that requires capital investment.”

But then the tap will be turned off and the flow of borrowing stopped in the second half of the coalition's term.

“You scale it back down so that the level of growth is above the level of deficit. That's what happened for the last six or seven years in the last recovery.”

Ryan admits that, first, major projects under the National Development Plan will have to be reviewed. And the three parties are on the same page on that.

“I think the National Development Plan is going to have to be reviewed for a whole variety of reasons. And a lot of the cost overruns, you know, the likes of children hospital has gobbled up a huge amount of money that might have existed and we have to look at.

"And even that may have changed in the last six months. Will Covid mean that the cost of construction inflation maybe stops? And I think the whole climate issue requires [us] to review the national development plan because it wasn't climate proofed, and we do have to meet these targets, and it's going to be good for our country. I think there's broad agreement that that will be done.”

Then there is paying back the debt. And reducing down the huge deficit from the pandemic has been at the heart of talks between the three parties: how to kickstart the economy again and yet somehow not drain all that fiscal fuel away with cutbacks and austerity taxes.

Ryan outlines how new broader taxes will help the new coalition pay back those debts over its five-year term. These have yet to be decided. But he maintains they will be progressive and spread wide. If true, that would alleviate fears among the party's grassroots over the deal.

“In terms of what's paired back as well, part of what's done is that deficit can be reduced in two ways. One of the other ways is to have tax revenues, and particularly if you can broaden your tax base in terms of so that it's it's not relying on, what might be cyclical.”

Ryan says aviation taxes, backed by Europe, will be one of those options.

“I think it's [the air tax] going to be one of the mechanisms of addressing climate change.”

The Irish Examiner understands that an air tax of €3 per passenger is being considered in the coalition talks - but only when the aviation sector recovers.

A financial transaction tax, as advocated by the EU, is also favoured by Ryan.

“There are ways you can do this that have benefits that would reduce the kind of some of the speculative bubbles stuff.”

Overhauling property taxes to ensure owners of higher value sites pay more would also help raise more revenue and pay back debt, says Ryan.

"And you might look at issues like site value taxation rather than the current domestic property tax so that there's a broader spread of property taxation on land. It helps you address the housing crisis”

A key demand for the Greens in the coalition deal is increasing public and transport and cycling and walking routes. And Ryan believes this can be done outside Dublin too.

I do believe in light rail for Cork, and I don't think we should be putting it off for another decade.

Plans by Dublin city to open up pedestrian and cycling areas need to be mirrored in other cities and under government, he said, as well as bus routes.

“I think we should start by taking some of those routes and putting in a really quick way to kind of good quality bus cars. And, and also walking and cycling facilities so that it becomes really safe and really attractive to move around that way. And do it quickly.”

Ryan is also optimistic about the Greens getting their demand to have 20% of transport budgets spent on cycling and walking routes.

There is much speculation about the make-up of a Fine Gael, Fianna Fail and Green Cabinet, with the latter expected to get three seats at the table?

Ryan also has not given up the idea about potentially being Tanaiste.

"We'll see. I wouldn't obsess about that. We didn't take it up in 2007. It was on offer then, but we just felt it wasn't essential. None of that stuff will be decided until the very end [of the talks]."

While Ryan admits to me that it is a “tough time” to go into government, he says the Greens at the Cabinet table will have the power to say no and dismisses speculation the two bigger parties could gobble up the smaller coalition partner.

Would he pull the plug on the Government if an issue was reneged on or promises not kept?

“Yeah of course, depending on how definitively important it is. Of course you would."

And the climate change target of reducing carbon emissions by 7% will be a "red line", he reiterates to me, in the coalition.

“We are too big to be swallowed up. Green politics ain't going away.”