These words can be read on the plaque erected on the wall of Torca Cottage, on Dalkey Hill. This house was the residence where Bernard Shaw spent much of his childhood - his parents forming a peculiar ménage-à-trois with George Vandeleur Lee.
The quotation, however, is not from one of Shaw's plays or novels, but from a letter written a few months before this plaque was made. Already in his nineties, Shaw had become an object of culture in himself and arguably, the man of the century. Thus, in April 1947 John G. Fitzgerald, secretary of the Dalkey Development and Protection Association, suggested that a new park adjoining Killiney Hill be named after the playwright. Shaw wrote a letter to Mr. Fitzgerald indicating that "it must not be called Bernard Shaw Hill" because "not only would that be a clumsy ugly title, but out of the question because the men of Ireland are mortal and temporal and her hills are eternal." With the sole exception of the use of "and" instead of "but," there is no question that this is the source of the quotation on the plaque.
The full text of this letter can be read on pages 793-4 of the fourth volume of Bernard Shaw Collected Letters (1926-1950)