Who in their right mind would want to defend hippies or to take them seriously? Like the character of Neil in the seminal Brit sitcom The Young Ones, hippies are unwashed, stoned and passively-aggressively out to lunch.

But 40 years ago, they had their moment in the sun.

This weekend is the 40th anniversary of Woodstock, the counter-culture music festival that put hippies on the world map and forever changed society's perception of rock festivals.

The festival showed that young people were capable of behaving responsibly. About 150,000 people were expected but instead over half a million showed up to form a small city on Max Yasgur's farm. There were just two fatalities – a heroin overdose along with a tragic accident, when a hippie in a sleeping bag was run over by a tractor.

By 1975, the year America pulled out of Vietnam, drugs, the Manson murders and cynicism among other things had undermined Woodstock's message. Hippies had become – and remain to this day – terminally unfashionable.

Not even a comedy genius like Arthur Mathews (Father Ted) could make them funny or likeable. But perhaps his underrated sitcom Hippies was a sign that the legacy of Woodstock was overdue for reassessment.

You couldn't argue with most hippie principles or ideals. The problems came when hippies entered the mainstream. Principles tended to get silly and ideals distorted. Just like when the Green Party shared power. As Homer Simpson said, hippies were expected to 'stick it to The Man', not become 'The Man'. Much of the anger of the punk movement of 1976-78 was directed towards hippie icons like Pink Floyd who were perceived as Establishment.

These days, it's easy to forget that Woodstock was a turning point, not just for American society but also for teens and young adults everywhere.

I grew up in the small fishing town of Howth, north Co Dublin (dare to call it a 'village' and you'd be run out of town). It was a beautiful place, but so dull it was almost spiritual. Even the local 'longhairs' were remote, mysterious figures. We longed for any distraction not based on activities that adults prescribed such as "going for walks" or "joining the sea scouts".

When the Woodstock film opened at the Carlton, it created an instant community of 'longhairs', 'freaks' and 'beautiful people'. We lapped up astonishing performances by Joe Cocker, Santana, Hendrix, Janis Joplin and the Who. The band that rocked my world was Ten Years After, led by Alvin Lee, a charismatic, fleet-fingered lead guitarist whose cherry-red Gibson 335 flaunted an iconic peace sign.

The music was epic, but the big revelation was the movie's depiction of American youth, who came across as articulate, perceptive and good-hearted. The kids in glorious technicolour up there on a movie screen weren't movie stars or politicians or preachers or poets. They were ordinary kids – just like us. And they had something to say. The shock was that they were being heard. Here was an alternative society that anyone could join. Well, provided you had long hair and wore denim or kaftans.

Anything was possible. It wasn't a question of dropping out. It was a matter of choosing one's own destiny. The film had a big influence on me and I imagine on many others of my generation.

Yes, Woodstock has a lot to answer for, and it's hard to forgive any festival that immortalises cosmic ninnies like Jefferson Airplane or the Incredible String Band. But Woodstock provided a small miracle of harmony in turbulent times. It gave a voice to the voiceless, and it demonstrated that everything – peace and love, even music itself – was political when you got right down to it.

Woodstock occurred at a time when music was an important badge of social identity, a coming-of-age statement as crucial and as memorable as first love or discovering what you wanted to do with your life (or what you didn't).

I wonder what would be the results should an enterprising film-maker attempt to capture today's festival-goers and music-makers at a weekend event like, say, Electric Picnic. Would our young have something to say? Do we have any idea about our young people's hopes and fears or how they feel about life in post-Tiger Ireland? And if they had something to say, would we care?

I have heard old fogeys of my generation complain that today's young people are "too compliant". They have had everything handed to them. They are virtually apolitical and have "no opinions" except perhaps about "year outs" to Thailand.

Virtually the same things were said about my generation (except that Morocco was the place to go). It was wrong then and I suspect that it's wrong today.

In 1969, the world seemed simpler. Today the problem is that there are too many voices – too much information from mags, free sheets, Skype, mobiles, texts, blogs, YouTube, Facebook, Bebo, MTV, a million other TV channels – that it's hard to know what young people really think about anything.

Woodstock's great legacy was not just the wonderful music. It rocked society by showing that its young people had something to say. And maybe Woodstock showed that hippies weren't so bad after all. I used to be one, and on days like today I think maybe I am still.

Happy birthday, Woodstock.

Michael Clifford is on leave