The story of how Lassar took the veil against the wishes of her parents, who wanted her to make a suitable dynastic marriage, is a common motif in the hagiography of Irish female saints. It is related that she entered the convent of the saintly Rígnach, who soon recognised the novice's potential for sanctity and learning. To complete her education, Lassar was sent to Rígnach's brother, the illustrious ‘teacher of the saints of Ireland’, Finnian (qv) of Clonard. It is claimed that Lassar became a distinguished scribe and that she was often seen writing in her cell, surrounded by flame. She was eventually dispatched to establish a convent of her own, her principal foundation being Clonlyon in Crích Lóegaire of Brega. Certainly, her cult became rooted in the Brega region. She would seem to have also been associated with Cell Lasra in Uí Bresail, apparently the townland of Brazil in the parish of Killossery, and perhaps also with Killester (Cell Lasra?) in the parish of Raheny, both in Co. Dublin.
Lassar is the subject of a rather late and derivative Life. She also features in the hagiography of several saints whose cults became more widely established, including Brigit (qv) of Kildare. In an episode that certainly appears anachronistic, it is claimed that Lassar was visited at her convent by Saints Brigit and Patrick (qv). As food was scarce, she allowed Brigit to miraculously increase what was available, and she subsequently surrendered the convent to her. Almost certainly, the episode is intended to account for the later dependence of Lassar's foundations on Kildare. A similar story, doubtless with the same intention of bolstering paruchial claims, is told of a visit to Lassar by Colmán Elo (qv), the saint of Lynally.
Several other saintly women of lesser stature named Lassar feature in hagiographical sources, at least some of whom are probably duplicates arising from cult localisation. The strongest claim to being a foundress in her own right would seem to be that of Lassar the daughter of Comgall, who is traced to the Dál mBuain of Ulster. She was perhaps the patroness of Cell Lasra (Killesher, in the barony of Clanawley, Co. Fermanagh). Lassar of Clonlyon, whom John Colgan (qv) believed to have flourished c.540, died on 29 March, under which date she is commemorated in the martyrologies.