The perennial challenge for even the most accomplished one-liner comic is how to vary the rhythm and tone of a whole hour of gags, lest the tried and tested setup-punchline formula becomes too routine.

Ryan Cullen has a bodycount of uncles in his jokes to rival that of Milton Jones' multitude of grandfathers. And the Northern Irish comic has always cultivated the darkest sensibility, a gifted writer leaning towards edgelordism in his more excessive lines.

True to form, he opens with a near-the-knuckle abortion gag and is soon knee-deep in paedophile quips, an Anne Frank bit luxuriating in its naked transgression. The relentlessness of his assault upon polite taste ensures that if one joke doesn't quite land, Cullen can simply shrug it off and move on to the next.

Yet after a while, he acknowledges the urn positioned behind him. Supposedly containing a loved one's remains – though who knows following the explosive manner of their passing – it's actually a repository for the jokes that he maintains have got him into the most trouble over the years, his greatest hits of depravity. 

And they're unquestionably that, tightly crafted pearls of casual cruelty and unrelenting horror. Getting audience members to pick them out tombola-style to read offers a patina of mixing things up. But in truth he simply repeats the gag if they fluff the line.

That's your theatrical conceit then, the minor variation on the formula you surmise. But not so. Although Cullen expresses the classic gagsmith's reluctance of not seeking to talk about himself, the person behind the cold, brutal delivery remaining hidden, he surprises with a sudden outpouring of anecdotes, about where he's come from and where he's at.

A health scare shortly before the festival may well have been the catalyst. And while Cullen derides such admissions in Fringe shows as grasping awards bait, he capably sustains his pitch-black tone while showing considerable vulnerability and engaging self-mockery about his subtly changed appearance. 

Thereafter, and with the dam now broken, the tales flood out of him, principally about his upbringing in Strabane, a somewhat lawless town on the Irish border. It was there that his father's Sectarian divide-bridging relationship with his mother got Cullen Sr badly assaulted by both sides.

The comic himself was shot as a child, albeit without life-threatening consequences. And when he relates his therapist's expletive-blurted incredulity at what he's been through, you're inclined to agree.

Yet Cullen's funniest tale is the explanation of why he can't do a regular job, another reason why he's standing in dark basements telling strangers Holocaust jokes. As a younger man starting out at a convenience store, he rolled his eyes at the owner's prejudice about the Traveller community and the zombie apocalypse-style manner in which he said they would set about shoplifting.

But the prediction came to pass, with Cullen recalling the hilariously precise details of how the prophecy was enacted, topping it with a grim postscript.

The storytelling acumen he's displaying here suggests  Cullen has developed another, substantial string to his bow as a stand-up while side-lining some of his more self-consciously edgy showboating, an impressive evolution this far into his career.