There was standing room only in a windowless arts venue in Belfast where a neuroscientist recorded the brain activity of a harpist as she played a melody about trees.The audience was enraptured.This was a lunchtime fringe event in the city’s Cathedral Quarter on the second day of Fleadh Cheoil na hÉireann.Playing the harp is a “neurological feat”, Dr Niamh Kennedy told us in a sweltering Black Box before she strapped on magnetic brain stimulators to musician Ursula Burns.“What we can see here during Ursula’s beautiful playing is that each of these squiggly lines represents a muscle, so you can see as she plays, her muscles are firing,” said Kennedy, pointing to a screen.Outside on the cobbled streets – once a no-go area during the Troubles – crowds were four deep as children busked outside pubs and restaurants.Tourists posed for selfies under illuminated umbrellas on Commercial Court, an alleyway that has become one of the most photographed in the quarter.Food trucks, stalls and “session spaces” lined the length of Royal Avenue, the city’s main thoroughfare, thronged with thousands of people by mid-afternoon.This is the first time Belfast has hosted the fleadh and much of the city centre has become temporarily pedestrianised.More than 800,000 people are expected to attend the event – double the number the last time the fleadh was in the North, when it was in Derry in 2013 – which will run until August 9th.Maurice Brown performed in the opening ceremony the previous evening with the Belfast Band Forum, an umbrella group for marching bands from the Protestant, unionist and loyalist communities.Maurice Brown of the Belfast Bands Forum: 'What a time to be alive.' While he was aware of some abuse on social media from “naysayers and keyboard warriors”, Brown said it was important to be part of the fleadh to “promote my culture”.The multicultural opening event – Dhol drummers and Indian dancers also took part – was “incredible”, he said.“With my previous military background, I would have done major state occasions but that event last night … what a time to be alive,” he said, beaming.Dancers Kerry Cudlip and Olivia Reid. Photograph: Seanín Graham “Had you said six months ago – even though the plan was well under way – that a Northern Irish Protestant march band would take part in the opening of the fleadh, I don’t even think Ards Comhaltas [the festival’s host branch], with their best intentions in the world, could have foreseen that. For it to go down the way it did was truly amazing.”Brown was at City Hall on Monday, where a main stage is the centre for headline acts and free concerts.“I just wanted to come today and have a wander with my family; I wanted to feel the atmosphere on the streets,” he added.Dominic McClean and his mother, Bronagh McClean. Photograph: Seanín Graham Hundreds of people covered the manicured lawns outside City Hall where friends Conor Newland and Matthew Walsh carried their bazooka and banjo on their backs.Newland, from Limavady in Co Derry, met Walsh, a Dubliner, at a fleadh three years ago.They agreed the event is “for everyone”. “Two of my friends I play trad with are Protestant; they understand. It’s not one-sided. It’s just Irish,” Newland said.Walsh isn’t familiar with the city and was still trying to find his bearings.“We haven’t played yet today; we don’t know where we’re going. That’s the beauty of it; we’re going to walk around, find a corner and sit down and start playing,” he said.“We met a few years ago and have played together since; that’s really the ethos of the fleadh. The main stages are great, and there are loads of acts booked in, but we’re looking forward to the sessions.”Louise Meleady from Belfast is looking forward to a joint session between a Lambeg drummer and bodhrán player. Photograph Seanín Graham Halfway down Royal Avenue, Louise Meleady stood in a queue for another event. The musician from Belfast has her own school, Ceathrú cheoil na bhFál, and is looking forward to a joint session tomorrow between a Lambeg drummer and bodhrán player.“I think it’s important that we talk about our histories, how similar we are,” she said.“From when I was young we used to play for McPeakes [a traditional Irish music school in west Belfast]. We were brought to the fleadhs every year and you always got that buzz with the crowds.“I was worried how it would work in Belfast, but literally when we came here yesterday morning, we couldn’t believe it. We got the same feeling.“It’s happening, it’s fantastic, the buzz is here.”