Ballagh's work has been exhibited at many solo and group shows since 1967, in Dublin, Cork, Brussels, Moscow, Sofia, Florence, Lund and others, as well as touring in Ireland and the US.

Born 22 September 1943,[5]: 5  Ballagh grew on Elgin Road in Ballsbridge, the only child of a Catholic mother, Nancy (maiden name Bennett), and a Presbyterian father, Bobbie (also Robert), who converted to Catholicism.

[3][5]: 96  His father was the manager of the shirt department of a drapery shop on South William Street; his mother, who came from a comfortable middle-class background, stopped working when she married.

[9]: 10 Before turning to art as a profession, Ballagh was a professional musician for about three years, initially with the showband Concord, then, on a full-time basis, as bass guitarist with The Chessmen, managed by Noel Pearson.

Around the same time, Ireland's Arts Council purchased an acrylic of a razor blade on canvas, inspired by a theory of critic Clement Greenberg.

[8]: 10  He next turned to political themes, notably connected to Northern Ireland[10] but also with elements inspired by the Civil Rights movement in the US and the reaction to the Vietnam War.

[5]: 7  He also produced three early works which have remained critically recognised ever since, inspired by Liberty at the Barricades (Delacroix), Third of May (Goya) and Rape of the Sabines (David).

[10] He was commissioned by his former tutor, Robin Walker, to produce abstract designs for screens in the new restaurant building of University College Dublin.

[5]: 8  Using the same concept, in his first major public commission, for the Five-Star supermarket chain's new shop in Clonmel, he painted a large-scale (c. 80 foot) mural on 18 panels.

[13] He also, drawing on scenes from The Life and Opinions of Tristram Shandy, Gentleman by locally-born Lawrence Sterne, painted a series of panels for a local restaurant.

[5]: 8–9 He later painted fellow artist Louis le Brocquy, writers J.P. Donleavy (2006), James Joyce (2011, commissioned by UCD), Sheridan Le Fanu, Oscar Wilde, Brendan Behan and Samuel Beckett, as well as poet Francis Ledwidge, singer Bernadette Greevy, politician Carmencita Hederman[5]: 33  and a rendering of Fidel Castro.

[5]: 53–54  One community art project in Dublin, to make a massive mural placed in front of the Custom House, was the subject of an RTE television documentary.

After he lost the lease on that in the mid-1980s (it was repurposed as part of the development of Temple Bar and eventually hosted the Walt Disney Company's Irish office), he worked from home.

[5]: 68  On another occasion, in 1994, he was commissioned to produce stamps commemorating five Irish Nobel Prize winners; four were released but the fifth was cancelled when An Post belatedly realised that the subject, physicist Ernest Walton, was still alive.

[8]: 17 For Riverdance, impresario Moya Doherty, co-creator of the show, asked Ballagh to use a hand-made approach, and he produced around 50 small images, which were then projected to form backdrops.

[8]: 17 Ballagh was also designer for the opening ceremonies for two major sporting events hosted in Ireland, the 2003 Special Olympics World Summer Games and the 2006 Ryder Cup.

[28] Many years later, he was commissioned to produce a tableau vivant for a promotional event at the Royal Hibernian Academy's Gallagher Gallery, a living artwork inspired by a famous painting – he chose to work with The Girl with a Pearl Earring.

[5]: 24  His first exhibition was in 1969, at the Little Theatre in the original Brown Thomas shop on Grafton Street; it was opened by Conor Cruise O'Brien, who described him as "an exceptionally gifted, thoughtful young artist.

[38] Further retrospectives followed, the next, and most significant, being at the Gallagher Gallery of the Royal Hibernian Academy, with a gallery-within-a-gallery format,[5]: 118  – with a display of his stamps at the General Post Office in parallel.

At the event he breakfasted with Gregory Peck, lunched with Yoko Ono and chaired a panel consisting of Norman Mailer, Gore Vidal and Graham Greene.

[43] He was a founding member of Ireland's national academy or "affiliation" of artists, Aosdána, in 1981, and its first chairperson (the leader of its presiding body, the Toscaireacht).

[1][28] He ceased active participation in the body in the early 1990s, after what he felt was undue pressure to declare his personal views in a debate about censorship.

[49] In an interview with The Irish Times, Ballagh ascribes his "political awakening" to hearing news of civil rights protestors in Derry, Northern Ireland, being attacked by the Royal Ulster Constabulary (RUC) in 1968.

[5]: 17 In July 2011 it was reported that he might consider running for the 2011 Irish Presidential election with the backing of Sinn Féin and the United Left Alliance.

He is an active member of the Ireland Palestine Solidarity Campaign, which has asked that artists and academics participate in boycotts of Israeli businesses and cultural institutions.

Ballagh condemned the hypocrisy of political leaders, saying: "I know arts funding is not a big issue for people struggling to put food on the table but we are talking about the soul of the nation.

[61] He released an autobiographical volume, A Reluctant Memoir, in 2018; it is not written as a chronological summary of his life but consists of a range of short pieces around major events.

[65] Ballagh met his future wife Betty (Elizabeth Carabini, from a Dublin family of Italian descent) in 1965, when she was 16 and he was playing a musical gig.

[64] Betty Ballagh fell and received a brain injury in 1986, falling into a coma and requiring an operation to remove a clot; it took her years to fully recover.

[5]: 71  He had chemotherapy treatment for chronic lymphocytic leukaemia, and recovered fully; he subsequently received a diagnosis of type II diabetes.