ASHBOURNE, Ireland — Until recently, Aoife and Muiriosa O’Sullivan, 23-year-old twins from a religiously conservative family, led nearly identical lives: They wore identical clothes until they were in their early teens, studied the same subjects in college and both went into teaching.

Yet, aside from a quibble over the virtues of veganism, they parted ways for the first time over what could be a defining moment for them and their generation: a divisive referendum on legalizing abortion, set to take place on Friday.

Aoife (pronounced EE-fuh) plans to vote for the measure, saying that a woman has a right to choose what to do with her body. Muiriosa is against abortion under any circumstance, saying she is opposed on moral and religious grounds to the taking of a life.

The pair, who grew up in Waterford, on the southeastern coast of Ireland, are hardly alone in finding themselves on opposite ends of a bitter and seemingly irreconcilable row. The argument over the referendum has exposed wide divisions among Irish women and has emerged to some extent as a debate among women for women.