The roots of the idea behind the term are actually quite old, although no one knows when, exactly, people first began to talk about Irish twins. In both England and the United States, a massive influx of Irish immigration in the 1800s led to a negative connotation with Irish people and society. This often happens when a large immigrant group begins to settle in mass numbers in a new country. The Irish were accused of being backwards and uncultured, and it was assumed that they were uneducated, dirty, and a general pox on society. As a result, the use of the word “Irish” began to be pejorative.
A number of derogatory terms incorporating stereotypes about the Irish began to emerge, including “Irish confetti” for thrown bricks and “Irish kiss” for a slap. Irish twins fits into this vernacular, and is actually insulting on multiple levels.
Firstly, the term pokes fun at the stereotypical fertility of Irish Catholic families, which traditionally do not use birth control. In addition, it implies that the Irish lack the ability to plan ahead or control themselves, having children in quick succession rather than responsibly spacing them. Finally, it suggests that the Irish do not understand the medical definition of twins, which involves two children conceived and born together.