The section of the Diocesan website for the Diocese of Christchurch in New Zealand has an entry for the Legion of Mary . Understandably (to any except the Irish?) this is what that entry says about the foundation of the Legion:
The Legion’s Origin
It commenced in Dublin in 1921, to serve God in the Lay Apostolate, with a New Zealander Elizabeth Kirwan as its first president. It has since spread throughout the world and reached Oceania in 1932, Dunedin in 1933 and Christchurch in 1934.
A google search using "Elizabeth Kirwan Legion of Mary" suggests that, in New Zealand, Legion premises that might in Ireland be called "Frank Duff House" are instead named "Elizabeth Kirwan House"!That Elizabeth Kirwan was the president of the first praesidium of the Legion is correct; she was also, later, the first president of the association as a whole after it had developed into several praesidia under a supreme council called Concilium. The pattern of what is now seen as the first meeting of a Legion praesidium was taken from that of a monthly meeting that Mrs Kirwan had been running for young girls in Dublin, a meeting that had been going for some four years before the Legion itself began. [Nowadays this meeting would roughly be equivalent to a parish or deanery young adults group, though with a stronger sense of prayer and apostolic work than might be common in such groups today.] Many of the features of the Legion meeting today - but, interestingly, not the altar which began as a spontaneous new feature of the meeting on 7th September 1921 - were inherited from the pattern of Mrs Kirwan's monthly meetings. Her sense of discipline can also be seen in the notion of respect for the integrity of the Legion system that continues today.
I suspect that Frank Duff would have quite approved of the idea of Elizabeth Kirwan as a co-founder of the Legion. Those who knew him report that he was uncomfortable with talk of himself as the founder. The secretary elected at the first meeting of the Legion on 7th September 1921 recorded Frank Duff's name as the first in the list of attendees. Frank Duff crossed that out (initially the group was going to be only open to women to avoid any sense of competition with the Society of St Vincent de Paul). Instead, we find in Frank Duff's own handwriting, an addition at the bottom of the minutes to say that "Frank Duff also attended the meeting".