Dave Fanning is shaking me by the lapels and yelling. He's not angry; he's laughingly illustrating an anecdote about when Gerry Ryan insulted the Manic Street Preachers at the IRMA awards. "They came walking through the hotel... 'Where's this guy Gerry Ryan?' Gerry was at another table talking to Frank Murray, manager of the Pogues, and he points over to me and they came over to me and grabbed me. [This is where Dave reaches over and shakes me.] 'What were you saying about us?' Morah Ryan got up and threatened them with a fork. Then they were escorted out by bouncers, order was restored and the next day the music papers were filled with 'Manics thrown out of the Irish Brit Awards!'" He laughs. "I didn't put that in the book, did I?"

No.

"I probably should have."

By his own admission Dave has left a lot out of his autobiography The Thing is..., published by Harper Collins. There's a lot to say. He's a rock institution. My teenage self, poised over the 'record' button of a double deck tape player to illegally record his show in the late 1980s, would have been pretty awestruck to know he'd be shaking me by the lapels 20 years later. Teenage-me would probably be less impressed with how long it takes to transcribe him. Fanning says more words in an hour than most people can say in 10, sometimes adding clauses and sub-clauses until sentences are paragraphs. (Many of the quotes I've used have been excavated from much longer pieces of spoken text.) He's pretty forthcoming. And yet the book is pretty guarded.

"I'd say it's very guarded," he says. "If I had it my way it probably would have been even more so. It wasn't really my idea. I really don't want details out there about my life. That's why I did the whole thing myself, writing it in biro and getting someone else to type it up. Then a guy was brought in as a ghost writer, and he said, 'We're going to change a few things.' He wanted more detail about my private life and my girlfriends between 17 and 24, that kind of thing, and I said, 'The reason that's not there is that I don't really want it there, so it's not really me to do that', and he said, 'You're going to have to do it; it's an autobiography.' So I said, 'Okay.' He came over to Dublin, interviewed me for three hours, and he put a lot more of that stuff in. Yer man would say, 'Well, was Ursula [his wife] your soulmate?' and I'd nod. I wouldn't use the word 'soulmate' myself but I let him put it in."

Unsurprisingly then, although paying lip service to the repressive Ireland of his youth, he claims in the book to have been personally unaffected by it all. So music wasn't an "escape" then? "No, not at all. I had nothing to escape from. But it was as important as a blood transfusion. It was vital to living." He recalls listening to Radio Luxembourg under the blankets. "The programming from seven to eight was like a religious experience... each one of the five days had a different theme tune; one had the guitar riff to 'A Whole Lotta Love'. It was essential listening... In those days, music had to be searched out. I think it's been cheapened now that people can find it so easily."

Fanning's love for music was never quashed by his religious mum Annie, or his easygoing civil servant father, Barney. As long as he went to college ("academia was the family obsession"), they were happy to have him blasting out tunes in his suburban bedroom in Mount Merrion, where he lived until his late 20s. "Well, that's the thing of being the youngest of seven," he says. "I was allowed do what I liked. I had nothing to fight against. It was just so easy."

He scraped through a UCD degree and did a HDip before landing a low-paying job as a music journalist with Scene magazine. "Niall Stokes had been editor but had gone off to found Hot Press. He was in shock to see Scene continue. He was furious! I don't blame him. There was a real rivalry for a while. But all's fair in love and war!"

A Scene interview with infamous pirate-radio entrepreneur "Captain" Eamon Cooke led to Fanning presenting a Wednesday-night radio show on Radio Dublin, where he eschewed the chart hits, mid-Atlantic accents and fake names of his pirate radio contemporaries, and used it as an opportunity to play his own record collection and the demos he collected from hopeful bands around the city. He did this first on Radio Dublin and then on the much slicker Big D. "The only time we were raided I was away at a festival with Joe Breen and Jackie Hayden. Big D also burned down at one point. And yet, when it reopened all the equipment looked mysteriously the same."

What are you suggesting? "I'm not suggesting anything!" says Dave laughing. "It was just weird."

In 1979, with the establishment of 2FM, Fanning was hired by RTé and has been there ever since. "I came for what appeared to be an audition," he said. "I was feeling nervous anyway, but when I took my first album out of its sleeve, it was a copy of Blonde on Blonde, this one was literally broken away at the edges like someone had taken bites out of it. I thought, 'Oh my god I'm not making a good impression.'"

He was given the midnight to 2am slot, but he was used to keeping later hours. "On the pirates you might start at 12 and you'd broadcast until eight just for the laugh," he says. "But I'm also convinced of this: people used to stay up later in those days. Eleven o'clock is the new 1am. Everyone is going to bed earlier. If, for instance, there's loads of traffic now at quarter past seven in the morning, well, it used to be not till half eight. So the corollary of that must be that people are going to bed earlier."

And bands were happy to come in at these late times, sometimes a little worse for wear. "Ian Wilson [Fanning's long-time producer] didn't drink but he sussed that there was nothing wrong with a bit of anarchy. There was a lot of twistedness and in one way it led to a better scene. Everything was live. I remember one night Madness came in with the Go Gos. They'd appeared on a pirate station just before and Ian went mad about this and kicked them out."

In the mid '80s, Fanning joined Gerry Ryan and Mark Cagney on the anarchic RTé-promoted tours of the country ('Beat on the Street'). "It was just madness. I remember in Cork we literally had an armoured truck to keep the hordes from us. Gerry was mad. He was like the singer of the band – climbing up the stacks, miming everything from Robert Plant to Freddie Mercury. We were being handed towels as we walked off stage. It was ridiculous and fantastic."

He says he still finds it hard to believe that Gerry Ryan is dead. He first befriended him when working at Big D in 1978. "Me and Morah [Ryan] used to answer the phones for Gerry's show. Four years later those two got married. I met Ursula in '86 and after that I always associate myself and Gerry with myself and Gerry and Ursula and Morah. It was always the four of us."

He sounds like he was a bit of a challenge to Ursula. In his autobiography he depicts himself as a sort of commitment-phobic perma-teen in a job that extended his adolescence. Is that true? "You have no idea," he says. "I was never less than a mile from the family home – Kilmacud national school, Blackrock College... UCD was 20 yards away; RTé was 400 yards away. I left home at 29 and thought that was too young. When I married Ursula at 36 I thought that was waaay too young."

Indeed, the week before the marriage he gave an hilarious interview to the Sunday Independent in which he said he wasn't sure he believed in monogamy and that if the marriage didn't work out that's what divorce was for. What did Ursula think of that? "She didn't care, but one of her uncles refused to come to the wedding." Despite his insistence that his whole career is an adolescent whim, his schedule often sounds like that of a workaholic. He juggles television, radio and writing commitments. At one point he was managing to do his RTé radio show along with programmes on Channel 4 and Virgin Radio. "I went to London twice a week for about 10 years... and yet never stayed there. The first priority every Saturday was to do my Virgin radio show and get back home." He would sometimes cheat by recording the last bit of his RTé radio show and then use that for the end of his Virgin radio show so that he could get the last flight home. "Nobody ever noticed," he says with a grin.

Outside of Ireland he's best known as a champion of U2 and last week hosted an RTé documentary advocating Bono as Ireland's greatest person. A close friend of the band from their early days, Fanning downplays his prescience in supporting them. "I knew them well before anyone else but I only knew they were good at about the same time as everyone else," he says. "I thought they weren't anything special then. I liked them as people though, and ended up playing them more often than any other band on pirate radio, no question about it."

Why did he keep doing it? "I don't know!" he says, sounding genuinely perplexed. "I have no idea. I suppose they were the band I'd been supposedly championing by that stage, and I wanted to take it all the way."

I ask him whether at this stage he's like an embedded journalist incapable of criticising U2. "Yeah. No question. I mean I can slag them, but when I look at the body of work... well, it really stands up."

He does have strong feelings about other things besides music. A simple question about the time he accidentally shared his views on Boyzone with the country (thanks to an open mike on a television show) leads to him diving into a critique of first media culture and then business malfeasance. "Today the papers are so sensationalist they miss the point. The other day in the paper there was a story about a young guy from Anglo Irish killing himself, then yesterday dick-brain Sean FitzPatrick was grinning away on the cover... 'I have no money but my wife has four million'. Sean f***ing FitzPatrick, you dickhead, this guy called Stephen Doyle aged 32 years of age is dead because he couldn't take the pressure you created. Ah, it sickens me."

The one hint of palpable anger in the book is when discussing how in 2007 he was criticised by the Broadcasting Complaints Commission for strongly expressing his negative views on the Catholic church during a review for The Marian Finucane Show. The book features recurrent criticisms of organised religion. Why is that an issue that exercises him so much? "Are you implying that I felt more oppressed by it than I realised?" he asks, but doesn't give me time to answer. "Well, possibly... but I never felt it stifling me. I just think organised religion is daft and ridiculous; dioceses and archdeacons and parishes and the Holy See... all with the hats on them, spewing smoke. It's just stupid."

Why isn't there more of that anger in the book? "Wait till the paperback edition," he grins. The hardback version, however, while an entertaining look at Fanning's career, eschews personal revelations and strong opinions, although Fanning clearly has the latter. "I'd like to do more talk radio," he admits when I ask him about his move back to a more music-based format on 2FM. "But it's not on the cards right now. I suppose in a way I'm back doing what I was 30 years ago. But at this stage no matter what happens my gravestone will say either 'rock dickhead' or 'rock guru'. No matter what I do I'll be the rock guy."