They used to call it Ireland's 33rd county. Kilburn went 'green' in the mid-20th century when Irish migration to north west London hit its peak. For the young men (typically) who came here to build roads and railways, this was a home from home. You couldn't beat the emerald isle, but at least the High Road offered a substitute — lined as it was with pubs, dance halls, and other diversions designed to swallow up navvies' earnings.

Flogging Molly eulogised that very street in a rasping punk ditty; Ian Dury named a band after it; and the IRA — it's alleged — openly fundraised on it. In the 1970s, Kilburn was infamous and a far cry from today's cultural hotspot.

The area in the modern day represents the changing face of London immigration: exciting and pluralistic, yes, but no longer just the territory of any single ethnic group. The stats aren't easy to collate since the locality falls between three boroughs, with the very High Road itself split in two. But the anecdotal evidence is that 'County Kilburn' was now the stuff of history; about as current as the lives of the ancient druids.

Is there any truth to it? Off the author went, in search of the lost Irish; dropping in that old Celtic surname of his to anyone who would listen to the full length of it.

The footy fans

Our first calling point is The Kingdom Bar. A green-and-gold-coloured boozer next to an Afghan carpet shop and a Filipino restaurant, it’s now just one of many international options in this part of town. It's named in honour of County Kerry's Gaelic football team, who today conquer their Dublin opponents in the Division One final.

The sport seems to combine the best bits of rugby and football, and all the crowd noise of both. The assembly bang and roar their answers at us. One Patsy Rush, a former carpenter who came to north west London from Monaghan 29 years ago, explains that he's here for the banter. "Kilburn's not actually changed for me because this place stays the same. It's all about the people." And the match? "Not even the match. Who's playing?"

But if the people have stayed the same, then what happens next? The Kingdom's landlady Linda Kissane, a second-generation Irishwoman, gives a theory that the economic resurgence in Ireland in the 2000s stopped the flow of new arrivals to London.

"It was always said that Kilburn was as far as the men wanted to walk from Euston with a suitcase," she laughs. "Now young Irish people can afford to travel further — Australia maybe. The older guys who are still here sometimes have a perception of failure. The pub is here to support them. It's what the Irish do."

The campaigners

Less of a success story at the moment is the closed-up Corrib Rest, a once-thriving Irish hangout towards Queen's Park. Kevin Barrett, 65 years a local resident, is trying to save it. "Things change," he concedes, adding that the venue's plight is not about a shortfall in Irish custom — to which it could have adapted — but an overall changing face of Kilburn, which developers have exploited.

The new Kilburn

So its population has changed, but it's the also very fabric of County Kilburn that's different now? We decide to make up our own mind by continuing our hunt. There prove to be several red (headed?) herrings.

Much like a St Patrick's Day party, Gaelic iconography is ubiquitous here even if Gaelic people aren’t. Attracted to the exterior of one shop, we make enquiries. The man behind the till, a smiley gent of Pakistani birth, is Muhammad Hussain. He admits the significance of the décor is "news to me", but wants to hear more about our research.

The commentator

Another slightly chance arrival is Kerstin Rodgers, who's ended up in north Kilburn. She needed a home spacious enough to launch her supper club: a project that's perfect, you suspect, for someone with all the hospitability her joint Irish-Italian ancestry suggests.

"But who isn't a bit Irish these days?" she asks. A food writer, she's delighted in the cultural intermingling she's witnessed in the area across a few decades, saying the navvies' caffs she knew in the 80s were lonely old places. "I'd probably like even more diversity and gentrification," she reflects. "Two great restaurants have just opened. That was literally last week."

The next generation?

Larking about in the sunshine, Wayne Smith and Keelta Higgins are probably having a nice afternoon before we collar them for a quick word.

Wayne was born in Kilburn to Irish parents. Like others, he's noticed the relentless march of the flat whites, and he claims his neighbourhood has "lost character" through gentrification. Keelta has witnessed that in just the seven years since she got here from Northern Ireland. Are both nostalgic for an era they never actually knew? "No," says Keelta, "2017 is the best time to live in County Kilburn. When you leave it, you do miss it."

The flock

Like a wedding latecomer in a Richard Curtis romcom, we're hovering awkwardly and conspicuously at the back of a church. It proves very worthwhile. After his service, Father Michael O'Connor proudly mentions that he regularly welcomes 64 different nationalities to Mass, and would we like to meet some of them?

Three of the four turn out to be Irish. Eileen Rea came from Limerick in 1971 or 1972, Bertie McCarthy swapped the homeland for hotel work in London in 1961, while Michael Donoghue made the move in 1958, to labour on the railways. Joining them is Richard Nash, a native of north west London and a son of Guyanese and Jamaican parents.

The cheery quartet share an accepting attitude towards change, recognising that Kilburn looks different to how it used to — but so what? "What you have to understand is that it's a positive story in many ways," explains Father O'Connor. "Those families had their time in Kilburn and then moved on. That's social mobility."

The local legend

So, perhaps we were meant to find Irish people, but only so many, and only in particular places — and this was the hope all along. After a day of searching and only sometimes finding, that upbeat message gives us some consolation. As does a pint or two of Guinness at the Sir Colin Campbell.

Tonight is Irish music night, and it's hard keeping the black stuff from spilling all over the newly-refurbished floors. Crowding around the performers, the punters all yell and stamp their feet; and there's so much stamping of feet, we worry nearby Cricklewood will feel an earthquake, or a tsunami will rise up out of the Hampstead Heath bathing pond.      

We get an introduction to 'banjo royalty', Mick O'Connor. Born around the corner in 1950 to two Irish immigrants, he's lived up there, along there, and around that corner. "I've only ever known Kilburn", he shrugs, speaking in an accent that's London all over. Maybe, we wonder, his beautiful music shows that knowing Kilburn isn't so different to knowing Ireland, because cultural myths are such powerful things, and that Irishness is really about how your play your banjo — not where you do it.

Again, a shrug. "Well, someone had to live here."