A much-loved Dublin voice fell silent last week. The Dun Laoghaire foghorn sounded its final blast on Tuesday. This may not seem like a big deal to you, but if you live along the south Dublin coastline, you'll feel like you've lost an old friend.

The foghorn kept us company on sleepless nights, bellowing defiantly at the sea. It reassured you that someone was keeping watch, sounding a warning during times of danger. It connected you to countless other sleepless people along the coast. There was genuine sadness when it was silenced.

Up the bay, in Ringsend, another watchman will go to bed tonight without the foghorn's lullaby. Unlike the foghorn, John Gormley slept through his watch.

Our generation looked to his Green Party to be our watchmen when we put them in the Dáil. They were to ensure that an outrage like Wood Quay would never happen again. They were to ensure that the ideals we held as teenagers – cleaner environment, cleaner government – would become reality.

Gormley repaid our faith by selling out again and again. On Tuesday, he sold out for the last time.

When news broke that Brian Cowen had golfed with Sean FitzPatrick, Gormley staged one of his well-rehearsed passion plays. Using the exasperated expression he keeps for really special occasions, he said he had found out all he could, but he was "not Sherlock Holmes". (He's not even 'Menolly Homes' although he's showing more cracks.)

Cowen may not have done anything technically wrong, despite what chancer David Drumm says. But here was a last chance for Gormley's Greens to say "enough is enough". We, the public, said it months ago.

Gormley is a power-at-all-costs merchant who traded his Green credentials for the illusion of power when he teamed up with the Soldiers of Destiny. They must die laughing every time one of his harebrained plans falls on its face.

Remember how he announced that there would be a direct Dublin mayoral election last summer? Has it happened yet? Remember how he called for a general election in mid-January? Has it happened yet?

Under Gormley, the Greens became a party with no integrity, propping up a party that had no integrity to begin with. They have compromised on every major principle they stood for. Before entering government, they supported the Shell to Sea movement, the halting of US military planes landing at Shannon and the rerouting of the M3. After entering government… Well, you know the rest.

Gormley's not even a good tactician. This was highlighted last year when he forced his stag-hunting legislation through as the drink-drive limit was being lowered. Both were seen by Fianna Fáil TDs as attacks on rural life. His stubbornness could have brought down the government he's desperately clinging on to now.

Gormley's legacy will be one of failure and disgrace. He will always be remembered as the man who forced the M3 through Tara/Skryne. The road is being tolled by a foreign company for the next 40 years. It's proven an abject 'turn-off' for motorists, with the result that the taxpayer is making up the shortfall in its revenue. Last October, this paper revealed that the road will cost the state €50m in penalties. Gormley had the power to stop the road being built but didn't.

The M3 is not the only example of his hypocrisy. While he tut-tutted over Fianna Fáil's expenses culture, he was hiding a dirty secret himself. In 2008, he took a ferry to Holyhead to appear environmentally friendly – and had a Mercedes drive all the way from London to collect him. When the conference he was attending ended, he flew home.

Of all the betrayals committed by Gormley since 2007, his worst has been to make 'green' a dirty word. He has made environmental idealism synonymous with double-dealing. He has damaged the cause a generation fought for. It may be years before it recovers. He wants to be seen as Mr Clean, keeping The Enemy (Fianna Fáil) at close quarters. He doesn't realise that he is the enemy. He's so delusional, he told a newspaper last week that he believes all his Green colleagues will be returned in an election, along with a couple of senators to boot. (A Green senator to boot? Can I boot Dan Boyle?)

To add insult to insult, Gormley claimed he was staying in power because he was committed to seeing the finance bill implemented. We all know he's really staying to squeeze as many Green-related projects through as he can – projects that will most likely be unpicked by the next government.

Gormley had one last chance to redeem green politics when Cowen's on-the-green politics were revealed last week. He didn't take it. Again, he refused to blow the final whistle.

He was supposed to act like that foghorn we once shared. Instead, he stayed silent as Fianna Fáil drove this country onto the rocks.

I don't know how he sleeps at night.