After carefully studying the habits of the fish and making himself acquainted with the old acts of parliament, he endeavoured to rouse public attention, with a view to legislative reform.
In 1834 he was appointed to the commission of the peace, and by firmness and tact obtained the full confidence of the people in spite of his tory politics.
In 1837 a petition upon the Irish fisheries was presented to parliament by the Earl of Glengall, a friend and neighbour of Ffennell, who spoke upon the subject in the House of Lords (19 June).
It was due to their exertions that an act was passed in 1842, embodying many of Ffennell's proposals, but unfortunately giving privileges to the stake weirs, which long hindered the development of the fishery.
His office included the inspection of sea fisheries, and during the Great 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.
Their report led to an act passed in 1861, under which Ffennell was appointed inspector of salmon fisheries for England and Wales.
His chief power lay in his practical knowledge of the salmon fishery question in its minutest details, and his singularly clear and effective method of bringing forward the subject at public meetings.