“I remember when I worked in a hotel bar, there was an old man who used to come in who had fought on the Somme. He never said much about the war but he was a very talented and well-read man. All I remember him saying about it was: “No sane man ever went over the top sober,” and that always stuck me, because what sane person would?” Sean McGinley asks me.

Seán has one of those faces, instantly recognisable, craggy, characterful, haunting, Irish, bringing back a myriad of parts – Braveheart, Gangs of New York, Michael Collins, Ripper Street.

That he has starred in so many Hollywood blockbusters is largely irrelevant to the 60 year old, whose only concern is the script itself.The chance to head up the cast of Frank McGuiness’s iconic war play Observe the Sons of Ulster Marching Towards the Somme, was therefore an easy decision.

“They asked me and it was simple: I said yes. Because for me it's always about the quality of the writing. It’s the most important thing.

“It’s also about a part of history that I didn’t know enough about,” he admits. This was soon rectified: the whole cast visiting the Imperial War Museum, and given a guided tour by a specialist on the Ulster Regiment, before rehearsals even began.

“It brought everything into perspective,” Sean tells me. “Just seeing the horror of what those men went through and the array of weaponry and ingenious ways to harm and destroy them and their fellow men filled me with a mix of awe and disgust.”

Perhaps even more poignantly, the play’s first performance was at the Somme’s centenary, at the Ulster Museum on the former battlefield. “It had a profound effect on us all. Everyone should go to because it changes your perspective for ever. We were looking down into the woods where the attack was launched

“If any one is in any way ambivalent about war, they should look at the monuments and the profound waste of talent and life evident there. The Memorial of The Unknown,” he tails off, “and then acres and acres of graves. It’s a memory I often go back to, and for what gain?. The best example of the monumental stupidity of our species,” he says shaking his head in wonder.

“And yet Frank McGuiness never visited so his plays was a great feat of the imagination and yet he got it absolutely right. So again, for me, it was a question of the writing rather than a need to get involved.”

Playing Older Pyper, the only survivor of 36th Ulster Division going over the top on July 1 1916, he returns to the past and revisits his lost friends and comrades.. “He relives what happens which is then re-enacted. But more than that the play touches upon the effects of war and the trauma rather than the battle itself, and this is their story, It’s one of the most iconic Irish 20th century plays

Born in County Donegal in the little village of Ballyshannon, Sean went to Galway University to do an art degree when he was asked to do a play with the newly formed Druid Theatre Company.

“Until then it wasn’t on my radar but I did it anyway and now here we are. That was in 1977, so 39 years later and with no training, it was happenstance. I have never taken it for granted. Every time I get a new job I feel like I have never done it before.

Often playing baddies, Sean said he doesn’t distinguish between good and bad “although characters like Richard III are ultimately more interesting aren’t they, even if he’s not necessarily someone I’d want to go for a pint with.”

Neither is he overawed by his stints in Hollywood: “They do a lot of time sitting around waiting for things to happen.”

Filming Braveheart must have been dramatic though? Yes Braveheart did get me excited. It was filmed half in Scotland and half in Ireland where the battle scenes were filmed and the scale and numbers quadrupled overnight. It was a bit of a military operation but overall it was a great experience working with some amazing people

But I like working so I like to cast my net far and wide, except for theatre it would seem?

“No, I haven’t done theatre for six years, because I was in The Republic, a six season CBC series filmed in Canada, not screened here, but I’m really enjoying it and the touring,” he says almost in surprise.”

“We rehearsed in London, opened in Glasgow then Liverpool, all places I really wanted to go and then we take it to Ireland, Cambridge and Oxford, so it’s a great tour with a great bunch of people.

"And I don;t find it tiring because engaging with something as high quality as this, is quite energising, even though the subject matter is quite harrowing. And it has its funny bits, humour being part of the fabric, their sense of humour being a real coping mechanism.”

As for the future, Sean is ambivalent, before admitting: “I haven’t done much Shakespeare. There isn’t a great tradition of it in Ireland. So every time I go to London I pop into The Globe to watch something. It’s such an eye opener to watch Shakespeare as it was supposed to be seen, and the interaction between the audience and the actors.

“I went to see a matinee of Richard III and when there was a thunder storm hit, and everyone despite the skies opening, it carried on and no one left. It was so wet it was like someone flinging a pint of water over your head. So if someone can hold your attention through that there’s something really special going on.”

Observe the Sons of Ulster Marching Towards the Somme at Oxford Playhouse until Saturday October 1. 01865 305305 or www.oxfordplayhouse.com