Its actually not a slur I am first generation Australian (from irish decent) and I've had that term used around me by many including irish people. It generally means because many irish families are large (my grandfather was one of 15, my granny one of 11) and to be large they often pop one out and have another shortly after So the name came about. One thing the irish are good at is laughing at themselves, maybe we all need to take a leaf out of that book