It’s hard to describe the sight of 80 000 people to someone who’s never seen such a thing. Especially when it takes the form of a palpitating mass, bending around the contours of Slane - with most of the participants roaring, drinking, good-vibing, clapping, climbing on each other and holding up burning things when the music suggests it. It’s the maddest sight.

And while this fizzing assembly has tolerated and even enjoyed the likes of Kelis, the Red Hot Chilli Peppers, JJJ72 and Coldplay, the U2 part of the night is unquestionably a supreme deal. An Irish band, once again the boss international draw, crowning the 20th anniversary of Slane as a venue, steering home on a rare shiny, summer day, the death of the singer’s dad still a raw memory. Just to tweak the atmosphere a bit more, somebody plays ‘The Boys Are Back In Town’ over the sound system at around 8pm, and the roaring intensifies.

Bono ambles onto the stage, gets down on one knee and crosses himself. Presently, the sustained whoosh of ‘Elevation’ vents the energy of band and crowd, leading smartly into ‘Beautiful Day’ as the peculiar tone of the tour (considerable joy, high-winging hope, some hints of despair) starts to bleed through.

Old tunes like ‘New Year’s Day’ and ‘Sunday Bloody Sunday’ sit well alongside new songs such as ‘Kite’, which has come to define the last days of Bono’s dad. Tonight, he changes the lyrics to remember Bob Hewson and his generation: "the last of the opera stars, when be-bop drove the big cars / the time when media was the big idea". The singer also notes that ‘A Kind Of Homecoming’ was written and recorded at Slane Castle, and so another jag of memory finds its expression.

U2 supported Thin Lizzy here in 1981, now they hail Phil Lynott’s spirit with a wobbly version of ‘Dancing In The Moonlight’, a song they doubtless played better in their very early days. But, hey, nice gesture. And they show some of their own worth when ‘Bad’ morphs through the Rolling Stones’ ‘Fool To Cry’, past ‘40’ and into an extra-fierce ‘Where The Streets Have No Name’. Enormous lights pan over the audience, and Bono lashes the people around, getting them giddy and tempestuous.

‘Bullet The Blue Sky’ is aimed towards the trigger-happy minority in Ireland. Bono spells out the names of those organisations and makes his aggressive-pacifist line clear. And with that, he gets progressively gentler. With ‘One’, the image of his dad is relayed over the video screens, a very public idea that some people may take issue with. But that’s the guy’s therapeutic instinct. He’s so out there, his life so magnified and buffeted, that he puts all his feelings directly into the ether, even tailing the song with a version of The Three Degrees’ ‘When Will I See You Again’.

The spiritual keynote is taken up with the closing song, ‘Walk On’, a kind of a benediction in this context. It’s a chance to cool down after the high pitch of the night, to pull some thoughts back in, and to get ready for a mighty show of pyrotechnics (that’ll be the unforgettable fireworks) hurtling vividly over the banks of the Boyne.

A bit of a great show, actually.