In 1844 an act was passed authorising police protection for the rivers; and in 1845 another salmon act was passed, and Ffennell was appointed fishery inspector under the board of works. His office included the inspection of sea fisheries, and during the Potato Famine he visited Scotland, examined the process of fish-curing, and tried to introduce it among the starving population of the west coast of Ireland. In 1848 the act commonly called "Ffennell's Act" was passed. This is the initial act of modern salmon legislation, which provides funds and machinery for carrying the law into practice, by making the local administration of the salmon acts self-supporting. He now became a commissioner at the Office of Public Works for the superintendence of the newly formed fishery districts. In 1853 he exhibited working models of salmon passes at the Dublin exhibition of that year, which attracted general attention. His advice was frequently sought in England and Scotland; and in 1860 he was appointed one of the royal commissioners to examine the salmon fisheries of England and Wales. Their report led to an act passed in 1861, under which Ffennell was appointed inspector of salmon fisheries for England and Wales. In 1862 he was appointed commissioner of fisheries for Scotland.