The Gods Weep, Hampstead Theatre, London<br/>A Good Night Out in the Valleys, Miners' Institute, Blackwood<br/>Sisters, Crucible Studio, Sheffield

Shakespeare this ain't, but it tries very hard: Would-be 'King Lear' has wit, courage, and an excellent cast, but a lame Jeremy Irons doesn't help

A young woman wants to cudgel Jeremy Irons to death. She's wildly thrashing with a big stick while he – ragged as a tramp – dodges to and fro behind a tree. Clearly, he's a rotter, and Joanna Horton's Barbara looks set to be his comeuppance.

The Gods Weep, Dennis Kelly's new saga for the RSC, is something like King Lear crossed with Enron – the West End's sardonic thriller about corporate shenanigans. At the outset, Irons's Colm is a sleek-suited businessman, cadaverous and faintly reptilian. The boss of a multinational, who alludes to his global operations as if they were satellite kingdoms, he suddenly announces he's dividing his empire but entrusting nothing to his son. Ruthlessly dismissing Luke Norris's feverish Jimmy as a soft touch – for loving his colleague Beth – Colm hands executive power to his cut-throat managers, Helen Schlesinger's Catherine and Jonathan Slinger's Richard. These two become viciously competitive. Meanwhile, their ex-boss goes demented, troubled by a guilty conscience, having destroyed Barbara's father years ago.

Alas, the principal letdown in Maria Aberg's production is Irons's lame performance, his bouts of madness particularly unconvincing. The drastic last-minute paring of the script doubtless hasn't helped him hit his stride, and to add insult to injury, the play's narrative momentum still slows to a snail's pace in the second half.

But then the mood is meant to be mellower. After the office machinations have surreally morphed into a savage armed conflict – all flak jackets and jugular-slashing knives – Colm and Barbara find themselves in a pastoral comedy. There they make their peace, share a makeshift tent, try to spear squirrels, and obsess over how to rustle a sheep before they starve to death.

The Gods Weep is weakest when it most closely echoes King Lear, struggling to vie with Shakespeare's poetry. Yet, for all its sprawling flaws, its ambition is impressive. Kelly has the courage to think big, and the piece is strewn with unnerving images. Especially memorable are descriptions of a devastated city where people sit behind bank counters or outside cafés, with faces blown away or flesh burnt black. Equally surprising flashes of satirical comedy lighten the mood. Two guerrilla fighters, munching on blood-spattered relics of Black Forest gâteau, discuss their former careers: "Futures and hedge funds." "No wonder you're so calm."

Irons finally hits a sweetly entertaining stride in the woods, making a pig's ear of hunter-gathering. Moreover, almost all the supporting cast are excellent, especially Schlesinger and Slinger: she ice-cool yet with a nervous energy, chain-smoking, watching like a hawk; he like some psychotic office nerd or dwarf cousin of Michael Caine, staring coldly through square-rimmed glasses.

In A Good Night Out in the Valleys, Kyle (Huw Rhys) is the capitalist invader. Prospecting for previously undetected minerals in a Welsh backwater, he's returning to his native land, surveying for a London-based corporation and planning to settle personal scores – until he falls in love.

Alan Harris's new play is also, significantly, the inaugural production for National Theatre Wales. Artistic director John E McGrath's new company aims to reinvigorate the Welsh theatre scene (with English-language productions) and serve as a flagship enterprise, with international scope.

What's most immediately winning about A Good Night Out, however, is its grass-rootedness. The show is set to tour miners' institutes and working men's halls (in Blackwood, Blaengarw and Bedwas), and the story centres on crumblies, couples and stroppy teens still gathering in these communal hubs to have a laugh and several pints, play bingo, swing punches, and sing.

Harris's play, loosely based on first-hand anecdotes, is a robustly humorous portrait of a small town, after the demise of the coal industry. But there is no danger of its rivalling Dylan Thomas's Under Milk Wood. With its busy, role-swapping cast, few of its characters rise above caricature. Harris also waxes increasingly sentimental.

Nonetheless, the cast's bounce is heartily enjoyable, with Sharon Morgan bustling up and down the aisles hawking sausages, and Boyd Clack's drunken, lardy Prior chased off the premises in his Y-fronts. Angela Davies' set design – with kiltered platforms and video projections – is experimentally stylish, while the characters try to find the right balance between nostalgia and new developments.

Last but far from least, Sisters is a verbatim docudrama put together by actress Stephanie Street, drawing on interviews with British Muslim women. Ruth Carney's studio staging instantly appeals; it's a get-together in a chintzy sitting room, with Street, Denise Black and three other lively actresses who chat and hand out pakoras and cups of tea to the audience as if we're all family.

The material isn't sharply structured and may not be proportionately representative. Yet it presents a fascinating variety of voices – aged 19 to 60, British-Pakistani, -Afghan, -African and Caucasian, footballer, religious scholar, lesbian and transsexual. All contemplate or passionately argue their faith, the veil, patriotism, and sexism. Some of the stories of tyrannical fathers and arranged marriages are horrific. Yet what comes across are the family reconciliations, and a complex mix of segregation and integration, militancy and magnanimousness. This should tour.

'The Gods Weep' (020-7722 9301) to 2 Apr; 'A Good Night Out ...' (nationaltheatrewales.org) touring to 27 Mar; 'Sisters' (0114 249 6000) to 27 Mar