A Roman Catholic, he was precluded from a fellowship at Trinity College, and turned to political economy and the chance of competing for the Whately professorship.
During the winter of 1853 Walsh temporarily took on the duties of deputy professor of jurisprudence and political economy at Queen's College, Belfast, for William Neilson Hancock, and in 1856 he was appointed by government an assistant secretary of the Irish endowed schools commission.
William Stevenson, Governor of Mauritius, then placed him on a commission inquiring into the civil service departments on the island; the work occupied nearly two years.
[1][2] Walsh specialised in monetary economics, and in 1853 published a lecture course, An Elementary Treatise on Metallic Currency.
It was praised by contemporary economists, including John Stuart Mill and Nassau William Senior.