As Tadhg O'Sullivan and I sit down to discuss his extraordinary documentary The Great Wall – a study of borders, literal and virtual, across contemporary Europe – new sorts of chaos are breaking out at Calais. David Cameron has just triggered fury by referring to "swarms" of immigrants making their way from Africa to the ancient channel port. The film could hardly seem more relevant.

“Well, there have been a couple of things over the last weeks,” O’Sullivan says thoughtfully. “All these things rhyme with the film. There was Calais. Then there was also what’s happening to democracy in Greece. There were a few days when I thought the film might become irrelevant. Will the Greeks overthrow the technocratic power of Greece and the EU? I would have liked that, but it might have made the film irrelevant.”

As it happened, the uneasy status quo remained and O'Sullivan's film retains its relevance and its urgency. The Great Wall presents itself as an oblique adaptation of Franz Kafka's allegorical story The Great Wall of China.

That puzzling piece relates how a structure comparable to the historical wall is built piecemeal to repel supposed invaders from the North. Travelling from Africa to Bulgaria to the City of London, the film argues that something similar is happening throughout modern Europe.

“The one thing I had to change is that, in the story, it’s the people of the north who are seen as invaders,” he says. “In ancient China, it was the Mongols. Now it’s all about fear of the south. That sort of thing was always used as a way of scaring children. Now it’s used as an existential threat to the state to scare citizens. This has been going on for a long time.”

Talk of swarms

Which brings us back to David Cameron and his talk of "swarms". The Great Wall is a beautiful, puzzling film, composed of long takes scored to choral music and swelling drones. It makes its point subtly. We see security footage of refugees trying to make their way through southern borders. We drift down the Thames and ponder the connections between London and even more ancient cities. The tendrils connecting these images are largely hidden.

“I was in Palestine a few years ago and made a film about a television station for Al Jazeera, and I saw how it was penned in by architectural power and exclusion and separation walls,” he says. “Those ideas had been bubbling away for a long time. When I came across the Kafka story, a light bulb went on over my head. I could talk about these things without being specific or analytical. Those documentaries are interesting, but that isn’t what I wanted to do.”

It would be invidious to reduce O'Sullivan's film to a collection of influences and forerunners. But he does willingly acknowledge a debt to Chris Marker and, in particular, to that French film-maker's unclassifiable meditation Sans Soleil. Enthusiasts for the wonderful films of Patrick Keiller will sense fellow feeling in O'Sullivan's work. The most conspicuous influence is, however, the singular Irish film-maker Pat Colllins. O'Sullivan worked as editor on Collins's Silence and agrees that they have developed parallel aesthetics.

“I didn’t study film,” he says. “I studied engineering. I came at it that way, but always imagined that film was where I was going to end up. I worked in Galway and got to know Pat Collins there. The technical background is very useful. I can figure out any machine. As you’ll notice in the credits, I do almost everything myself.”

Both Collins and O’Sullivan employ gently avant-garde techniques to explore the hidden secrets of people and places. There is something of the London author Iain Sinclair’s psychogeography in their approaches. There are more to these city walls than mere bricks and concrete.

Evolving styles

“There’s a dialogue between myself and Pat that has allowed certain styles to evolve between the two of us,” he says. “What has been amazing working with Pat has been the ability to develop those approaches.”

Dig beneath the surface and the logic behind O’Sullivan’s strategy begins to emerge. We might be surprised to see footage from a control room operating under the umbrella of Frontex, the EU agency that manages cooperation between national border guards, but O’Sullivan argues that it was at least as difficult to shoot in the supposedly public spaces of the City of London. That fact tells its own story.

“Whatever else you might say about Europe it has an overarching ethos of transparency,” he says. “That’s the foundation myth of the EU. Frontex are actually brilliant about allowing access. They see what they are doing as a humanitarian thing and they are not entirely wrong. Getting access to detention centres is a very different matter, however.”

So, what was going on in the City of London? It is just one corner of the busiest metropolis in Europe. Can the bureaucracy surrounding film shoots be quite so (with apologies) Kafkaesque?

“The City of London itself is an almost entirely autonomous, bordered territory within London,” he says. “So that fitted in very well with the general feeling of the film. With a little bit of creativity, paying for access and planning ahead you can pretty much pull it off. But that actually caused more logistic problems than, for instance, shooting on the Bulgarian border.”

More than anything else, the film is about power. It’s about the hard power that expresses itself through barbed wire and detention centres. But it is also concerns the more insidious classes of oppression that emerge through propaganda and misinformation. All of that would be familiar to Kafka. That author’s name has, after all, been adjectivised into an expression of bureaucratic oppression.

“He would have been writing at a time when the threat to empires was growing,” O’Sullivan says. “The Austro-Hungarian Empire was collapsing. You were on the verge of the first World War. He cut through the specifics. What was prevalent to an observer was the dynamics of power. He wrote about how power works. That’s why I think he’s the most relevant writer for today.”

As a man of central Europe – a Czech writing in German – Kafka might have appreciated the film’s last image: chairs in an empty room.

“That’s the German Finance Ministry. Is that a spoiler?”

No, I don't think we're dealing with The Silence of the Lambs here.

“That’s right. Ha ha ha.”

Chilling, nonetheless.

The Great Wall will play at the Irish Film Institute, Dublin, from August 21st, 2015