If there is any upcoming comedian who can fly by the seat of their pants as well as Jason Byrne I've yet to encounter them.

In the past Ross Noble and demented Scottish stand-up Phil Kay have excelled at freeform hilarity, but this manic, majestic Dubliner in full flight is truly a sight to behold. Of course it all depends on a willing audience and for a few minutes last night things were wobbly. But when Byrne asked someone if they were from London and they cryptically replied "sort of", we were off.

With a posh elderly couple arriving late, a lawyer who had no idea who the star was and a skeletal man with a complicated private life, the crowd offered pure comedy gold.

And this unstoppable livewire certainly made use of it, seamlessly interweaving-improvisation and script. He was particularly good miming the way Tube commuters do anything to avoid contact - "facing each other and still not looking at each other".

If his male versus female patter was predictable, he was better on his guilt-ridden Catholicism, repeatedly returning to the subject of couples trapped on their very own "hate train".

Parenthood provided potent punchlines, too. Byrne's seven-year-old son, clearly as hyperactive as his dad, is a "drunken tramp", smashing everything in his path. In a riff that has developed brilliantly since last year, we heard how the lad was convinced that his baby sibling was the reincarnation of Steve Irwin. The updated pay-off has to be heard to be believed.

After the interval, the mixture of material and mayhem continued. If his appearance at the 2006 Royal Variety Performance fell flat, at least it has given him a sublime anecdote about meeting Prince Charles.

A section on national euphemisms for love-making petered out, but some Tommy Cooper-style magic brought things to a fitting climax.

I would recommend Byrne unreservedly, with one crucial caveat. This show depends as much on the audience as the artist. Yet if he can get a reticent crowd going on an autumnal Monday he is clearly on form.

The evenings might be getting cold, but for lovers of comic lunacy, this is the hottest ticket in town.

• Until 22 September. Information: 0870 890 1101, www.nimaxtheatres.com.