I came across the phrase “Irish Twins” for the first time today, used on one of the genealogical Facebook groups I’m a member of. I’d never heard it before.
“Irish Twins” is used to refer to siblings born in the same year. For example, one in January and one in December of the same calendar year. It is also sometimes used simply to refer to siblings born within a 12 month period, even if not within one calendar year.
It’s origins come from around the time of large numbers of Irish immigrants to the United States, and it started as a derogatory term implying that the Roman Catholic Irish, who didn’t use contraception, were unable to plan to have their babies more spaced out.
These days it just appears to be used as term to refer to babies born close together, and in fact many couples are deliberately choosing to have their children as close together as possible.
If I look through my Unst Family Tree there are many families where the babies are born close together, and where the term “Irish Twins” could be applied, but I suspect the term wasn’t known or used in Shetland in the 1800s and that it was quite normal to have children as nature took its course. In fact, when you see a family with gaps between children, or a gap after marriage before the first child was born, you are more inclined to wonder whether there were some problems with the pregnancies that undoubtedly happened in those gaps.