He was compared to Bob Dylan in the '60s -- and Dylan was scathing about it when they met. He helped Paul McCartney with the lyrics of 'Yellow Submarine' and in 1968 studied Transcendental Meditation with The Beatles, actress Mia Farrow and Mike Love of The Beach Boys at the Maharishi's school in India.
He taught John Lennon old-fashioned guitar finger-picking, which John subsequently used on songs such as 'Dear Prudence' and 'Julia'.
Linda, his wife of 40 years, had a son named Julian by Brian Jones of the Rolling Stones before they met.
Oh -- and he had a string of pop hits himself. You still hear them on the radio -- 'Jennifer Juniper', 'Hurdy Gurdy Man', 'Catch The Wind', 'Mellow Yellow' and 'Sunshine Superman'.
You could fairly say that Donovan is a genuine pop legend. Now he's coming out of retirement from his hideaway in Mallow and is set to go on the road again.
He explained: "I was at a Paul McCartney concert in New York's Radio City Music Hall earlier this year and Ringo Starr got up to play with him as it was his 70th birthday.
"I spoke to Ringo afterwards and he told me he still gigs with his own All Starr Band. I asked why and he told me that he's a musician, that's what he does.
"That kind of sparked something in me. So I'm coming out again. And I'll have a new album and show called Ritual Groove as well as the old hits."
Donovan and Linda, appeared on the Late Late Show when they moved here 20 years ago.
He recalled: "I remember Gay Byrne telling us not to be strangers and not to disappear down the country where we wouldn't be seen or heard from again. But that's what we did.
"We settled there and raised our kids. We live in an old rectory house and put it on the market at one stage, but we eventually decided to stay put.
"I also recently bought a local 18th Century church and we're doing it up to use it for different things like dance classes and alternative healing sessions, all sorts of things really.
"Oh yes, we settled in very well in Mallow. We have a huge extended family now that stretches right to America. I have five grandchildren now.
"My daughter Astrella is quite a skilled singer and is living in Los Angeles. She's a bit like Enya or Kate Bush and goes by the name of Astrella Celeste.
"I've been keeping busy myself with my artwork, which I call Saphographs. They're all in black and white and I'll be opening an exhibition of them in a gallery on Dawson Street in October.
"My dad was also a photographer and I recently discovered his old primitive camera. But it no longer worked. So I bought a mint condition 1964 Rolleiflex to work with."
When he first emerged in the mid-60s, Donovan was compared to Bob Dylan. His song 'Catch The Wind' was closely compared to Dylan's releases and Bob was suitably withering to him when he first came to London.
Donovan now laughs about it all.
"I think they got confused. Bob was something to watch and he certainly blew everything open. But we were really chalk and cheese. I think I sounded like him for about five minutes! I actually haven't met him or seen him perform in years.
"Back then I did need to meet him, so I got Joan Baez to introduce us. She also introduced me to Arlo Guthrie, and to this day we're still like brothers."
And Donovan also revealed how he helped his then rivals The Beatles to write two of their best known songs.
The Sunshine Superman, who had moved from his native Glasgow down south to London to follow his dreams, became pals with all the pop stars of the era, including The Beatles, The Rolling Stones and more.
The singer, who will perform in Dublin's Olympia Theatre on October 6 with a new band composed entirely of Irish musicians, recalled giving Paul McCartney some small help with the writing of 'Yellow Submarine'.
"One Sunday morning in London while I was writing a song there was a knock on the door and when I opened it. Paul was standing outside.
"I lived in Maida Vale and he was in St John's Wood. Outside I could see the sun beating down on the Edgware Road, and no cars on it. People didn't drive that much in those days.
"Anyhow, Paul asked me what I was doing and I told him I was writing some songs. He said he was too and asked to come in. He was writing Yellow Submarine, but was having some trouble finishing the lyrics. So I gave him the line 'Sky of blue/Sea of green/In our Yellow Submarine'. He told me that was perfect and finished the song.
"While it was ostensibly a children's song that Ringo sang, I knew exactly what it was about. The Beatles were so popular at the time it was like they were living in a bubble.
"They couldn't go anywhere without being mobbed or followed by fans. They were sort of living in a submarine, occasionally coming up for air.
"Then there was another knock on the door. I opened it and a young policeman was standing there. Paul come out to look over my shoulder and the guy jumped, saying 'Oh, it's you, sir.' He actually saluted Paul!
"He asked him if that was his Aston Martin down the road, parked with one wheel on the pavement and three on the road, with the windows open and the radio blaring.
"Paul said it was his all right. The bobby then asked him to give him his keys and he would park it properly for him and bring them back. Which he did!"
Donovan remembered: "I actually helped John with some of the structure of 'Julia', which was a very sad song. It was about the tragic death of his mother, who was knocked down and killed by a drunken, off-duty policeman while she was crossing the road. John was a young boy who was only getting to know her properly as he had been raised by his Aunt Mimi up until then.
"I also helped George (Harrison) with 'While My Guitar Gently Weeps'. It's based on a descending pattern based on a Bach piece. I just passed it on. I simply had some forms at the time that they didn't."
Forty-two years after his Indian adventure, Donovan still practises Transcendental Meditation and tries (in vain) to explain the four levels of consciousness to me. He works in America with the David Lynch Foundation which has introduced so-called 'Quiet Times' into US schools. "It helps to relax the central nervous system," he claims.
George and John left suddenly after falling out the Maharishi. Ringo had left earlier as he didn't like the spicy food and his supply of baked beans had run out. John later penned a song called 'Sexy Sadie', which slammed the be-robed holy man.
"The Maharishi was just a human being, but John nearly ruined his reputation.
Donovan and Linda still live in their beautiful old rectory house in Mallow.
He describes it now as "an empty nest," as all their children have long since left. But they do come back to visit with the grandchildren.
And the extended family includes children from previous relationships in the grand old hippy tradition.
"Linda had a son named Julian by Brian Jones of the Rolling Stones before I met her. They split up and Brian eventually died, as you know. I adopted Julian when I married her and he's got his own son now, who is great on the guitar.
"He's only 10 or something and the other day I showed him the riff to my old song 'Hurdy Gurdy Man' and he picked it out immediately. I was astonished. But I suppose I shouldn't have been, given his musical pedigree and background."
Donovan and Linda have two children together, Astrella and Oriole Nebula. The singer also has two children by his American lover Enid Karl, who he left to take up with Linda. He had loved Linda from afar when she was with Brian Jones. His son by Enid is named Donovan Leitch, while his daughter Ione Skye went on to become an actress.
In the 1990s Shaun Ryder, the then wildman singer with druggy band the Happy Mondays, took up with Donovan's daughter Oriole. So what was it like having Shaun Ryder as a son-in-law? Did he ever have to sit him down and give him a good talking to?
"Ha, no I didn't. They were a wild bunch all right, but I never gave him a talking to. Himself and Oriole had a child, but they're not together any more. And my daughter Astrella was with Shaun's brother Paul for six years.
'I remember going to Manchester and they [the Happy Mondays] called to my hotel room to see me. They took out screwdrivers and started unscrewing my door as they were leaving. They thought it was a lark and what rock stars did. I had to tell them that it wasn't on.
"But I enjoyed the Manchester 'scene' and can recall Liam Gallagher of Oasis telling Shaun he was going to be 'just like him' when we were in a pub one day. At the time Liam was a complete unknown and Sean just ignored him.
"Thankfully drink and drugs were never a problem for me. The problem with drugs is not wanting to come down from them. I never liked it. And I never once took heroin.
"I found hash good for the nervous system and would occasionally still have a smoke. I went for a check-up recently and the doctor told me I was in perfect health. I was quite surprised. I think that's because of the healthy vegetarian life we lead in Cork. I think I have an idyllic lifestyle down there."
Donovan's dad Donald, who had Irish connections in his past, took him to the registry office in Glasgow and told the clerk he wanted to call him simply Donovan, an Irish surname.
"The guy said he couldn't do that, that I had to have two names. So my dad, who worked at a distillery at the docks, simply slipped him a half bottle of whiskey and he agreed to it. Now I'm still one of the very small group of people who are known by just one name -- Madonna, Prince, Pink . . ."
The singer also believes that he was helpful in turning our own Christy Moore into a major star. "This photographer friend of mine called Tom Collins, who I would dearly love to meet up with again, introduced me to Christy and Planxty. I took them on the road with me in the early '70s and they went down really well. I still think of them as The Beatles of Irish folk music!"
Besides doing some European dates with his new band over the coming weeks, Donovan has also been asked to speak at a music fair in China and will soon perform the whole of his classic 'Sunshine Superman' album in London's Royal Albert Hall.
"I asked Linda, who has always been my muse, what she would like for our 40th wedding anniversary present and that was what she picked. It was inspired by her. So that's what I'm going to do."
Donovan and his new band will play the Olympia Theatre on October 6