"Irish Twins" is a slang expression for siblings born less than a year apart from each other. The term is often used to describe siblings born within 12 months of one another, or children who would be in the same grade of school. However, children born within 18 months of one another are still often referred to as Irish Twins .
The phrase "Irish twins" originated in the late 19th century as a disparaging term associated with Irish immigrants to the United States. While some people intend it affectionately, it does stereotype Irish culture, making it mildly offensive.
Here's an example: If a mother gives birth to a baby girl in January and then gives birth to another baby girl in December of the same year, those two babies would be considered Irish twins.
