For those not getting why you're being downvoted despite being factually accurate (meaning:you), allow me to explain.

First off, you appear as a rude, depressing scold for wanting to ruin people's momentary chuckle by introducing troubling racial issues as a topic. There are forums where that is appropriate and necessary. But not all fora are equal. Life protip: an elevator ride with co-workers would be another example of an inappropriate forum for this topic. Just in case you were considering another "joke".

Second of all, you have confused "ruining other people's smile" with the thing called a "joke". No, see, a joke is literally the exact opposite of that.

But your true violation is not being funny. See, if you had the slightest comedic instinct, you would have written But no *Irish allowed amirite" because the exclusion of Irish was very real, more successfully captures the distinct flavor of the prejudice of the Depression era, and since it's no longer really a thing, seems more absurd to the modern audience. Absurdity is surprising and would elicit what we in the jocularity biz refer to as a "laugh."

I hope this helps ... someone.