The other day in the store a woman walked by, saw my kids sitting in the cart next to each other and said "Oh, what cute kids! Are they twins, or irish twins?" After an awkward moment of stammering out some sort of answer, she then explained "Irish twins means that they aren't really twins, but look a lot alike." After she left I turned to my husband and said, "That's not what 'Irish twins' means."
So, lest anyone else accidentally insult a stranger while trying to compliment their kids, you should know that Irish Twins is a rude way to describe two kids born to the same mother within 12 months.