Many posters attribute it to Alexander Trevelyan, chief secretary for Ireland during the Great Famine, as his explanation for how the population came to be doubly decimated in the 1840s on his watch, as it were.
I use the phrase doubly decimated in the literal sense: decimation is the killing of one in 10; double decimation is the killing or at least the removal of two in 10. And that is the fate that befell the Irish population in the 10 years between the censuses of 1841 and 1851.
The total population of the island, as measured by the census of 1841 was 8,175,124; in 1851, the population had dropped to 6,552,385.
Do the maths, as they say (6552385-8175124)/8175124 = -19.8%, or in round terms a drop of 20%. Double decimation.
The report of the 1851 census is available online and two very interesting pages are this one and the subsequent page which contains the tabulated data.
Spanning the two pages is this wonderful phrase, written presumably by the Commissioners to whom was delegated the task of collating and analysing the data.
"The numerical decrease of the inhabitants between 1841 and 1851, amounted to 1,622,739 or 19.85%; but this.....conveys but very inadequately the effect of the visitation of famine and pestilence, with which it has pleased an all-wise Providence to visit Ireland."
The author was probably William Donnelly, the Registrar-General of Marriages, who had been appointed to "superintend the enumeration of the population of Ireland" although he was assisted by an Assistant Commissioner, one William R Wilde of Westland Row and one Edward Singleton acting as secretary.
Wilde? Westland Row? That rings a bell

Never mind the Fag on the Crag; this guy was the **** at the Count.