Grace Dent on TV: Mrs Brown's Boys, BBC1

Irritating relatives, terrible jokes, mindless banter: the perfect accompaniment to a family Christmas

Brilliant TV is apparently the cornerstone of Christmas, but in reality most of us spend the holiday missing all of it, sat in a living room thick with brussels-sprout wind and children's squeals, with the TV screen obscured by the fat arses of guests who've over-stayed their welcome. Did you enjoy the Christmas Doctor Who special? Or was that when Aunty Bridie needed her run home or your odious cousin began paraphrasing the best bits from his Mock the Week – Too Hot for TV DVD.

Other poor souls might have experienced a fate even worse: the entire family gathering as one – on the insistence of one of the more bossy house elders – to watch the Mrs Brown's Boys two-part Christmas Special. For those untouched by this Irish comedy phenomenon, Mrs Brown's Boys is the brainchild of comedian Brendan O'Carroll, who plays foul-mouthed Irish matriarch Agnes Brown, a feisty, foul-mouthed individual who spends her life meddling with the lives of her six kids. Once seen, it is rarely forgotten. To love Mrs Brown, and millions of people do, one must be thrilled by a man in a hairnet and dinner lady tabard saying the F-word roughly once every ten minutes, egged on by a loyal studio audience so whipped to hysteria by him that one can hear pants being soiled and spleens exploding with mirth.

The Christmas special opened with Mrs Brown inexplicably stuck on the top of a Christmas tree. "I slipped!" she said to camera. The audience erupted in happiness. "Wake up ya bastard!" she hissed at Grandad asleep in a chair, throwing a bauble at his head. Uproarious laughter. Stretchers gathered at the theatre's door to transport audience members to waiting ambulances. I have never laughed at this show once and by God, I've tried. I've sat there staring at it, mumbling, 'Concentrate, there are funny things happening there. People swear by the hilarity of this show'. But it's not on my brain LOL-frequency. However, I watch Friends re-runs every single day, so who is the arbiter of taste here? And there's a web page called sadtrombone.com which plays a comedy sound-effect of a Tom and Jerry "wah-wah-wah" honking "failure" noise which has been amusing me solidly for over three years. "Decorate the tree?" Mrs Brown said, surveying the living room from her position up on high. "Well it's not going too fucking well is it?" Not fecking, not ducking, not bucking, but fucking. If this was a show about young people called "Mrs Brown's Boyz" the plethora of blue language would have caused wild gnashing of teeth. Not so here.

The show is set around the Brown family home. It's a TV set, and Mrs Brown continually talks to the camera and also the studio audience. Cast members, of which there are at least 70, with names like Buster and Winnie, loiter into scenes, slamming doors which wobble, fluffing their lines and retaking, laughing at Mrs Brown's jokes when they shouldn't and holding their limbs at awkward angles like people who have never acted in their lives.

On first sight of the show, it's common to feel a slight televisual sea-sickness; nothing is as it should be by orthodox sitcom standards. It is a festival of wrongness. Typically, there will be a very slender suggestion in each 30 minutes of a plot; for example, in the Christmas Eve special, Mrs Brown had been conned to believe that the house had been fitted with secret cameras for a reality-TV show, so she had begun talking in a posh accent. Don't worry yourself with the vast unfeasibility of this idea. In fact, don't use your brain at all. We'll have no brains here today, thank you!

Mrs Brown's Boys is a very broad comedy. The brand sells millions of DVDs and Mrs Brown live tickets and brings light and joy to people's lives. It is Two Pints of Lager and Packet of Crisps, but more than ten times as crass and not as cerebral. It's that sitcom in BBC1's Extras that the Ricky Gervais character was embarrassed to be the start of, despite it bringing him adulation and wealth. It's a big, silly, thick show – part pantomime, part It's a Knockout, part 1980s sitcom Bread, part CBeebies. It's the show that the BBC have been dying to commission for years, but couldn't find anyone from their usual flock of writers who'd turn in anything as knowingly base. Because – and I mean it most sincerely – it's a skill to write something as bold, ragged and wholly cherished by millions as Mrs Brown's Boys. If you think you can do it, please grab a pen now and begin: there's big money to be made.

As several people have said to me, they know the show's flaws, but can never truly hate it when it makes their parents laugh so hard; so they all sit and watch together. Mrs Brown's Boys: the worst comedy ever made, yet the most Christmassy thing on TV.

Grace's marmalade dropper: The Queen's Speech in 3D! Woooo, ma'am, it was like you were in the room!