Spanning portraits, landscapes, still lifes, abstract works and text works, influenced by art history, linguistic philosophy, formalism, modernism and everday life, Brennan's oeuvre is rich and diverse. It is also beset by contradictions: at once cerebral and tactile, her works suggest both the rigour of her ideas and the gap between these ideas and the sensual materiality of the paintings themselves. Her abstract works often establish conceptual rules, while at the same time undermining these, as Judith Pascale has written, through the formal irregularity of the lines and forms and the lushness of the paint.

However all of Brennan's works attest to her mastery of, and joy in, colour; with a marked attention to the relational aspects of colour, her palettes range from sensitive to bold, from bright to sombre, from soft to schematic to vividly beautiful to lurid. As Max Delany has noted, her paintings are also permeated by a clever, wry sense of humour, playfully parodying "the regulatory authority of words and reason". Above all, perhaps, they suggest what Delany has called "the determined intermingling of art and life, and of life as an intensely aesthetic experience."