Nessa Ní Shéaghdha was born to Seán Pádraig Ó Séaghdha and Kitty Nic Caochlaoich on 14 March 1916 at 20 Aran Road, Drumcondra, Dublin.

She trained under Osborn Bergin and Gerard Murphy, completing a degree in Old Irish.

She started working in the Leabhair ó láimhsgríbhinigh project, which was established in 1937 and aimed to provide "readers of modern Irish" with texts which had not been printed previously.

Ní Shéaghdha also edited and translated a number of texts of histories of Ireland and mythologies including Tóruigheacht Dhiarmada agus Ghráinne.

Ní Shéaghdha also gave lectures in the Universities and acted as an external examiner.