A graduate of Dun Laoghaire Institute of Art, Design & Technology, she has had seven solo exhibitions including the Ashford Gallery (RHA); The Molesworth Gallery, Dublin; The Linenhall, Castlebar; and Dunamaise Arts Centre, Portlaoise. Una has exhibited in group exhibitions in Ireland, UK, San Francisco and New York, and has been a regular exhibitor at the Royal Hibernian Academy (RHA) for the last 18 years winning the award for a Work of Distinction by an Artist of Promise in 1993. She has also been the recipient of several prestigious Arts Council awards.

Una lectured in Painting at IADT from 1991 to 1996 and over the last 12 years she has been an invited tutor on drawing courses in the National Gallery of Ireland and more recently at the RHA. She was invited artist-in-residence for Éigse Carlow in 2009.

Sealy says of her work "My paintings focus on life as observed around me, often depicting family, friends and surroundings in North County Dublin, or further afield while travelling. I work mostly directly from life, preferring to be in the presence of whatever it is I am painting. My style has been described as a kind of "Kitchen Sink Realism" (Brian Fallon, Irish Times 1991) , an apt description I think".