Brendan Gleeson and Diane Keaton in Hampstead

THERE is, film star Brendan Gleeson says, something in the story of Harry Hallowes, the hermit of Hampstead Heath, that appeals to us all – something especially poignant about a man who wanted for nothing, who lived simply, getting one over on a billionaire who was trying to wriggle round planning laws to demolish a historic house and build an £80million mansion.

You will recall that Harry lived in the gardens of Athlone House, Hampstead Lane, and won “adverse possession” – also known as squatters’ rights – in the High Court to be allowed to stay in his ramshackle home. He died last year, and now a film called Hampstead, a rom-com whose lead is a man loosely based on Harry, is in the cinemas.

Gleeson, who plays a character called Donald living in circumstances that mimic Harry’s, has become a big-name star for his roles in films such as In Bruges, Calvary and as Mad-Eyed Moody in the Harry Potter franchise.

In the film, bailiffs seek to evict the hermit and Brendan says he liked the idea that in a “fairy tale love story” there was still room to consider vital issues over ownership of land, house prices and whether it is possible to live outside what society considers “normal” today.

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“My character’s point of view, when people were putting a price on his land, on his plot, is that money is not the issue – it’s the same bit of land as it was yesterday, and will be the same tomorrow,” says the Dublin-based actor.

“It is a piece of ground with my home on it and that is what it is worth to me – he doesn’t see it in financial terms. I like that – the idea of profiting from homes to me seems immoral anyway. The idea of providing or withdrawing shelter from someone in order to make money is just a crazy way of living.”

Gleeson added he hoped the land, as rumoured, would be given to the Heath or left to the Woodland Trust.

“That is a lovely legacy and a nice lesson for us all – and it is a great ending for a movie,” he added.

“The big thing is the fact the cost of the land means nothing. There is a lesson in there.”

After Brendan had first read the script by American writer Rob Festinger, he tried to contact Harry – but was told by a Heath Ranger who knew him that he wanted nothing to do with the film.

Harry’s response shaped how Brendan approached the role.

“There is an integrity about the film,” he says.

“I was at first worried that it would lack integrity, but it has worked. Above all, this is a fairy tale. My character, Donald, falls in love so we had to make it believable that Diane Keaton, [who plays a well-heeled Hampstead widow] falls in love with him. My first thoughts were I had to get rid of the notion that this wouldn’t happen. There had to be elements of Harry, but I know what it is like to live under canvas – I did plenty of that as a young man, and it is hard.”

Harry Hallowes, whose lifestyle on the Heath inspired Hampstead

In the film, Donald’s living conditions are positively luxurious compared to Harry’s – a point taken on by Gleeson as part of making the storyline more believable.

“There had to be an element of antisepticness applied when we made his home – we couldn’t make it like Harry’s, really,” he added.

Gleeson said he drew on the experiences of friends who lived alternative lifestyles.

“There are many people who live off the land who I know and they do it in a variety of different ways – they live frugal lives in the full knowledge of what they are doing, and I think Harry was like that.

There is a sense of dignity, of irascibility against the forces of conformity and I carried the torch for him as much as I could.”

Gleeson’s career has taken him from working as a teacher in Dublin and appearing in theatre productions to being sought after in Hollywood. And he believes that to keep the performing arts buoyant and diverse, we need to understand and celebrate their role – and not undermine their role in the curriculum.

“We do not respect the arts enough,” he adds.

“There is this lip service to the arts, but nobody wants to invest in them. It is odd – people do not mind investing in research for technology, knowing full well that it may not work at first, but you have to speculate to accumulate.

“But there is a great pressure on the arts to always produce things that are immediately both critically and economically successful, and that to me is fundamentally wrong.

“The French have a better attitude – the arts are seen as utterly central to who they are, and I hope we can get to that place too. The arts have to be recognised as a part of the everyday, not some optional extra. It is crucial – the arts make use feel less alone, it has an incredibly effect on mental health, and its social value is immense.”

• Hampstead is currently on general release.