'It's not usually this busy on a Thursday," apologises Dan Boyle, senator and chairman of the Green Party, as we walk-and-talk through the Millennium Wing of Leinster House. "It looks beautiful but it's terribly designed from an environmental standpoint," notes Boyle. Over the next hour I accompany him from a Seanad debate on data-protection, to a Q&A session with UCC political students, to a meeting with a company that converts elephant grass into biomass (all the striding is very West Wing). Boyle is an interesting figure. Although a senator, to all intents and purposes he's "a sleeping member of parliament", he says. Heavily involved with Green Party policy, he helped negotiate his party's coalition with Fianna Fáil and recently helped renegotiate a new programme for government, garnering an extra junior ministry. Even acting on his own, Boyle has had more effect on government than most of his Dáil-based colleagues, most notably when an angry tweet helped end the ministerial career of defence minister Willie O'Dea. Independent thinking started early for Dan Boyle.

"I was from a politicised family and we were always encouraged to ask questions and speak our minds," he says. "My parents emigrated and I was born in the US. We came back when I was about eight. I had a funny accent and my father stayed in the States for a while working in the merchant navy, so it was initially a difficult transition – I got into a few scrapes. But it wasn't that traumatic, I was comfortable after six months or so."

A possibly more traumatic transition happened in his teens, when he first embraced Green politics. He laughs. "My family were Fianna Fáil," he says. "My father was chairman of Micheál Martin's cumann. My grandmother had first editions of the Sunday Press up in the attic and halfway up the hall there was a line drawing of deValera. As a teenager I reacted against all that. There was something about the Irish political system that I didn't find particularly inspiring. I liked the Greens in Germany. They were starting to be successful and I looked for the Irish version, which was very small. I corresponded with the person who was the voluntary general secretary of the party and then after a few obnoxious letters by me, I convinced myself I should join. That was the late '80s. There was one town councillor elected in Killarney and the membership was at around 200, I think. But my parents were always supportive and proud of what I achieved. They even joined the party."

The Greens have come a long way since the 1980s (although with dwindling public support and constant dissent, some argue that they're headed back the way they came). It's now almost three years since they entered government for the first time. "We expected Fianna Fáil would be voted out of office but we hadn't ruled them out as a partner," says Boyle. "That's one of the myths that was created subsequently."

Boyle describes the 2007 negotiations with Fianna Fáil as "a bit surreal". He and his Green colleagues, John Gormley and Donal Geoghegan (then party general secretary) sat across a table from Fianna Fáil's Brian Cowen, Seamus Brennan and Noel Dempsey. It sounds like a humbling experience.

"You're talking about two very different political cultures," says Boyle. "There was definitely an arrogance in the way that Fianna Fáil approached it. There were patronising comments – 'You're playing senior hurling now!' – that sort of thing. We weren't technically required for a majority and that affected our negotiating strength. The two ministries were very important to us, because ultimately it's about being in a position to implement Green policy. It would take the review two years later to get the policy issues better highlighted." He laughs. "Fianna Fáil were more defensive the second time. They needed us. The arrogance was gone."

While the Greens found themselves beckoned like Faust into the corridors of power, Boyle himself lost his seat in Cork South Central. "I got exactly the same vote I got five years previously," he says, "but the pecking order had changed. I had also suffered from, strangely enough, positive press. People had the impression that I was all but elected and it hurt me in the end. But thankfully Trevor Sargent wanted to keep me on board and I was very grateful to have been ultimately selected as a senator."

How does he respond to criticisms that the Seanad is an undemocratic awards system and should be abolished? "I think it's important," he says. "I've said several times that I think the quality of the debate is better. The independent universities, strangely enough, given the democratic issues, give it a gravitas that isn't always there in Dáil debates. There's less point-scoring. Personally, I'd like to see senators publically elected via some sort of regional list system. I've been criticised because I'm not elected. But I did want to stay in public life. I could have gone backroom and got paid more, but politics is part of my own make-up. I've been involved for 19 years and I didn't want to leave it behind."

Indeed, Boyle seems to live and breathe politics. "I spend three nights in Dublin, three nights in Cork and because I'm party chair I'm usually one night somewhere else – a constituency group or a meeting of European Greens – it does mean that the minority of my life is at home. My family situation is very complicated at the moment. My wife [Bláithín Hurley] and daughter live in England, so I'm on my own basically, although my sister and mother also live in Cork [his father died 10 years ago] and I have good support. Political life can be difficult on your personal life."

Does he take any time off? "I've been trying to make Sundays available for myself... but only recently. I used to be quite obsessive, but after 19 years in politics I can see that's not so healthy. Sundays I've put aside for a long walk and I've joined a gym. Politics has taken a toll on my body in a very unhealthy way. Music is still a major stress release [Boyle was once a bass-playing singer-songwriter – he still, he says, thinks of someday releasing an album]. I've an out-of-tune piano at home and after a hard day I belt the hell out of it. Last week I was in Cork and for one reason or another I couldn't sleep and started playing piano at three in the morning – which must have greatly amused my neighbours."

Another place in which he sounds discordant notes is in the world of Twitter. Back in February, after the Greens helped pass a vote of confidence in minister Willie O'Dea (after O'Dea defamed, then denied defaming in a sworn affidavit, Sinn Féin councillor Maurice Quinlivan), Boyle tweeted that he had "no confidence" in O'Dea, declared him "compromised" and reopened an issue that Fianna Fáil thought closed.

"I think Sean O'Rourke on the News at One had more of an impact on his resigning than I did [O'Rourke's charged interview with O'Dea possibly convinced Fianna Fáil that public opinion wasn't with them]. But I was furious. There hadn't been time to think or react. The decision to put the vote on the order of business was made with the click of a finger. And the debate itself was a pantomime – the kind of name-calling and finger-pointing politics that I hate."

How did his party colleagues respond to his social media intervention? "Some were annoyed with me," he says. "But I don't think John Gormley was. I think he thought that if I was concerned, well then there must have been something wrong. We are a party that encourages and works with dissent and part of my role as party chair is to reflect the opinions of the membership. The day of the vote I'd been getting phone calls, emails, and texts and could see that what was happening was something that the party was really uncomfortable with and was something we couldn't and shouldn't have been associated with. I suppose I do have a freer hand in some ways than my cabinet colleagues. And Twitter's a great tool. Yesterday, for example, I made a statement about Charlie McCreevy being asked to join the board of Ryanair, which could be embarrassing for Fianna Fáil. We were probably over-worried early on in government not to seem like reckless people trying to bring down the government and I think maybe we didn't state our own position enough. As party chair, I think it's important to put Green Party opinion out there."

This concern with reflecting the party opinion is probably wise. The Greens have had a rough year – from the comically foul-mouthed eruptions of Paul Gogarty to the more serious issue of Trevor Sargent's resignation after his inappropriate intervention in a legal case. "Coming so close after the Willie O'Dea thing, I felt partially responsible," says Boyle. "I felt there was an element of come-back in it." Then there's the public perception that the Greens have been too willing to toe the Fianna Fáil line. Dissent on the issue has already led to high-profile defections. Some, like Patricia McKenna, were to be expected – she'd long quibbled with the Greens' stance on Europe. Others, like that of senator Déirdre de Burca came as more of a surprise.

"Déirdre was a good friend to me, we were very close, so I was very sad to see her go," says Boyle, but he then goes onto dismiss her criticisms as misdirected career-disappointment. What of the belief, widely shared, that the Greens shouldn't be propping up the compromised architects of economic disaster?

"But I don't think a change of government will change anything!" he says. "There's very little difference between Fianna Fáil and Fine Gael. The policy agenda is pretty much the same. They're going to have the same approach to public expenditure... the same approach to banking [Boyle insists that the supposed differences between different parties on issues like Nama really amount to the same thing]. I think there's an onus on us to provide as strong a government as possible. I know there's discomfort about this, but I think it's important to show we can be a party of government in the most difficult times."

Does he worry about the electoral damage that this position could ultimately do the Greens? "Opinion polls suggest that we'd get the same amount of first preference votes as in the last election," he says. "But we've stopped being a transfer-friendly party and that's a problem. Fianna Fáil come with a lot of baggage which the population and media react to. There's a credibility issue and the Greens are apparently guilty by association. But we believe the government is taking the right approach and that the country needs stability. And in a way I like when people write us off. In 2007, I was tearing my hair out because the press were giving people the impression I was elected before a vote had been cast. I prefer low expectations... that way I can come out fighting."