How will you answer the religion question on your Census 2016 form?

There is an old joke that people from outside Northern Ireland often tell. We are asked to imagine that a gormless tourist has wondered into one of Belfast's more belligerent quarters. A sectarian thug approaches and asks our hero what religion he practises.

“Oh, I’m an atheist” the chap says. The hooligan leans forward and breathes beery breath. “Are you a Catholic atheist or a Protestant atheist?

The anecdote is supposed to reveal a crucial flaw in the mechanism of sectarian division. But Northern Irish people understand perfectly what the boor means. Do outsiders think that the ancient divisions between sects are really to do with transubstantiation or the Immaculate Conception?

Don’t be silly. They are about far more important things, such as who is allowed to stomp rudely up whose street in the summer months.

In this place we recently took issue with Atheist Ireland over that organisation's attitude to the 1916 commemorations. I am, however, happy to confirm that it speaks sense on tomorrow's census.

Too many people who practise no religion still tick the “Roman Catholic” box on that form. Quite a few “Protestant atheists” do something similar (insofar as the phrase “quite a few Protestants” means anything in the Republic).

Is it asking too much for people who don't believe in God to admit as much? Do they feel that here is where they register their attitude to Pascal's wager? "Aha. Harry Mangan has an individual septic tank and gas central heating," God says, while peering over that citizen's shoulder. "What's this? 'No religion'? He's been keeping that a secret. No eternal bliss for him!"

What size is your trunk?

Here’s another thing. The question should be “Do you have a religion?” not “What religion are you?” Answering “no religion” to the first question is like answering “none, as I am not an elephant” to the question “What size is your trunk?”

Anyway, despite these undeniable truths, people in the Republic do still have some faint understanding of the distinction between Protestant atheism and Catholic atheism. The inclination to tick "Roman Catholic" may be linked to a fear that, just as Aoife has disingenuously completed her First Communion to permit entry to St Bufton's, the authorities will inform the hierarchy of your true beliefs and condemn you to education in a hedge school.

But it is more to do with a lingering belief that the words “Roman Catholic” or “Church of Ireland” or (yes, I mean you, Hilda Blennerhassett) “Methodist” describe a wider social condition that is only peripherally defined by one’s attitude to the Reformation.

The cultural distinctions between Catholic and Protestants in Northern Ireland – which have always offered grim illustration of Freud’s narcissism of small differences – are played out on a grand and depressing scale every marching season.

You can tell the communities apart by what hats they are wearing or not wearing. The worst pretend that they can tell the "other lot" by the set of their shoulder. The best recognise that what binds the two tribes together (mordant sense of humour) is far more significant than what separates them (17th-century fluvial conflicts).

If we are to believe the 2011 census, more or less everybody in the Republic is a Catholic; a staggering 84 per cent still ticked that box. A tiny 2.8 percent registered as Church of Ireland. Mrs Blennerhassett's parrot brought the number of Methodists up to 0.15 per cent. And an admirable 6 per cent told the bloody truth and registered as "no religion".

All this suggests that it’s hardly worth discussing the cultural distinctions between Catholics and Protestants in unoccupied Ireland. Those of us – atheist, agnostic and believing – who were raised in the Protestant religions are part of a rough demographic that is steadily going the way of druids and snake handlers.

Yet most people have, in their heads, a caricatured notion of the southern Protestant. The younger ones may cunningly call themselves things like Oisín and Clíodhna. But consult birth certificates and you find parents named Rosemary, Herbert and Kenneth.

As a friend of mine once said, referencing her own husband from across the (not really) divide: "They eat disgusting stuff like Marmite and brag about not having enough to pay the gardener." Bits of The Irish Times are used to stuff up gaps in leaking windows and clean up messes left by stinking spaniels. They have never had to buy furniture.

These days not many such archetypal Protestants remain. Marry within a community this small and you will soon end up with more seven-fingered descendents than is decent or healthy.

Never mind. Let’s hope that those no longer sincerely adhering to Catholicism use census day to vote the other sects towards more honest representation.

In other words, stop with the bearing of false witness already.