Séainín Brennan is a Northern Irish actress. She is known for various lead roles in TV, theatre and film. Brennan starred opposite Philip Glenister as Frances in Ronan Bennett's political conspiracy thriller Hidden, a role which saw her named Woman of the Year in the Arts by the Belfast Telegraph. She is recognized as one of the most notable stage actresses from Northern Ireland.

Brennan was born in Belfast to Tom Brennan and Maura Brennan (née Rafferty). She grew up on the Malone Road in South Belfast, and at the age of 8, she starred in her first play. As a child actress, she starred in various musicals, plays, television adverts and then as a television presenter. She attended Methodist College Belfast and then the University of Ulster, where she graduated with a BA Honours degree in European studies. Brennan earned a scholarship to attend the College of Europe in Bruges, where she graduated with a master's degree in European political administration. After graduating from the College of Europe, Brennan started working as an intern at the Secretariat-General of the European Commission. Then, she became a political advisor at European Strategy, a Brussels-based consultancy firm. She speaks Spanish, French, and Italian.

Brennan then left Brussels to train at the Guildford School of Acting in Surrey and the Film Actors Studio in Dublin.