[3] He returned to Ireland about 1844 with a Ph.D., and after heading to Dublin was appointed as assistant to Robert Kane (who had also worked with Liebig), Director of the Museum of Economic Geology.
His first-known scientific paper was for the Philosophical Magazine (July, 1845) describing his discovery of a more sensitive method for the detection of mineralised phosphoric acid.
In 1846, he became the Museum's official chemist; this position was terminated after two years after his criticism of the Society's poor choice of crops for his experiment, but his work continued and was part of the National Exhibition of 1852.
He founded and edited The Journal of Progress in 1851, containing articles on the application of science for the development of Irish industry, including contributions from Kane.
[3][4][5] He was a member of Young Ireland and a shareholder in the short-lived nationalist newspaper of 1848, The Irish Tribune, founded by three medical students, which was regarded as a successor to John Mitchel's The United Irishman, and which was similarly suppressed by the government and its founders arrested.
However, he did not participate in the Young Irelander Rebellion of 1848 as he was suffering from a long illness due to rheumatic fever, which had the life-long effect of weakening his heart.
In 1857, Sullivan read various papers on chemistry at the meeting of British Association, which met in Dublin: through this he received grants to research high-temperature salt solutions.
During his tenure, which was a residential post (Kane's presidency was largely in absentia), he engaged wide support and instigated major changes: the library, museum and botanical gardens were added to (including a glass house), a physics laboratory and engineering workshops were opened, the Medical (Windle) Building was completed (1875), the (William) Crawford Observatory was built with funds from the eponymous benefactor (1880) and female students were first admitted (1885).
[9] This necessitated a long introduction and appendices for the series, encompassing the whole initial volume, and became his most celebrated work, a key reference book in itself: Henry Maine, for example, quotes extensively and Jeremiah Curtin visited him for a few days in 1887.
Along with the future British ambassador to the U.S.A, James Bryce, and Richard Barry O'Brien, he was an editor of Two Centuries of Irish History (1888); he wrote the section covering the years from the Treaty of Limerick (1691 -1782) entitled The Ireland of the Penal Days with his friend, the poet and political journalist George Sigerson.
A memorial in the form of a celtic cross was installed on his grave, and the remainder of the public money collected for such was donated to the Munster Dairy School.