1950 The Irish Twins. "Noun. Irish twin (plural Irish twins) (rare, slang, offensive) Either of a pair of siblings born less than 12 months apart, especially if born within the same calendar year or school year or born one year apart."
My sisters were "Irish Twins" as far as I am concerned. Not sure if they fit the "technicality" of the definition, but first came one, and then came the other. They were 10 and 11 years older than me and 15 and 16 years older than Janie. We lived vastly different lives. But they were my keepers until they grew up and flew from the nest. They are quite different in personality and life experiences. So am I from them. They and my brothers took exceptionally good care of me while they were around. I'm lucky about that.