Costello graduated with a BA in Classics and History from University College Dublin in 1982.
[3] She presided over High Court bankruptcy proceedings involving Seán Dunne.
[4] She served a term as chair of the judicial wing of INSOL Europe, a federation of insolvency lawyers.
[5] In 2018, she made a reference to the Court of Justice of the European Union arising out of an action taken by Max Schrems regarding the EU–US Privacy Shield.
[8] She is the daughter of the former Attorney General of Ireland and President of the High Court Declan Costello and the granddaughter of former Taoiseach John A.