It's a question of expectations indeed. People want all the accoutrements of a modern, middle class lifestyle. If having children, or more than one or two children interferes with that, then they won't have more children. It's also a question of expectations in terms of what they want to give to their children.
Birth control methods mean they have a choice.
Religion and/or different cultural attitudes is a confounding factor in the case of immigrants from third world countries, for example.
In the U.S. minority men (African-Americans and Hispanics), for example, are, according to the statistics, much less likely to be willing to use condoms. Young women are also more likely to get pregnant outside of wedlock or a stable partnership and support their children through public assistance.
The more educated the women, the more likely also that she'll put off child bearing, meaning there will be fewer children, as the biological clock does tick on.
Ireland has traditionally been an extremely observant Roman Catholic country for all of its history up to very recent times. I think one reason was that religion was tied to national feeling and identity in contrast to the Protestantism of the controlling English and Scots. The result is that church laws were much more strictly followed than in other Roman Catholic countries.
When we arrived in the U.S., my parents were shocked at the size of Irish families. It wasn't unusual for them to have six, seven, eight children. It was said it was because they wouldn't use the "artificial" methods of birth control, proscribed by the Church. I'm a little skeptical. "Natural" methods might not work perfectly, but if you were practising them, how could you wind up with eleven or twelve children? Even today, while families of that size are uncommon, it's my impression that couples where at least one spouse is Irish tend to have at least three or four or even five children.