As quoted, this passage comes from Dayan Dr. Isidore Grunfeld's introduction to his translation of Horeb, one of Hirsch's major works, Introduction, lxix. Although he is generally very careful about quotation, Grunfeld gives no volume or page number for this passage, merely citing one of Hirsch's "most trenchant essays".
I cannot find any passage that exactly fits. But I will post here one
that is very close -- so close that I conjecture that it is the one that Grunfeld had in mind, and that he was quoting from memory.
The original was published in 1860 in Jeshurun, entitled “Aw: das Zion der Zukunft und die heutige Reform”, accessible via Digitale Judaica-Sammlungen der Universitätsbibliothek Frankfurt http://sammlungen.ub.uni-frankfurt.de/cm/periodical/titleinfo/2945015, and it was reprinted in Hirsch's Gesammelte Schriften VI. In the English translation in Hirsch's Collected Writings, vol. 1, 390=391, it reads as follows:
“Perhaps the most radical cure in a time of confusion and the loss of all values would be the – provisional – closing of all Synagogues! Let such an idea not frighten the reader. The closing of all Synagogues would not affect or alter the precepts of the Divine Law one iota. But even a hypothetical announcement of the removal of the prohibition against eating the gid ha-nasheh, the sinew of weakness, for example, would shatter the very foundation of God’s Law! The closing of all Synagogues through Jewish hands would constitute the loudest protest against the denial of the Divine Law in life and home; it would give the most drastic emphasis to the truth that Divine Judaism embraces and dominates the totality of Jewish life and does not find its fulfilment in the halls of prayer and worship. God’s Temple and His Law must rule and shape our daily lives – this would effect the lasting cure for all our ailments. Then perhaps, we would again be worthy to regain our Synagogues and ultimately even the ‘bloody’ rite of sacrifices at Zion.”
This is from one of Hirsch's essays on the month of Av, when the fast on the 9th commemorates the destruction of the Temple. His point is that the Temple was destroyed because Jews lost sight of its higher purpose -- the localization of Jewish aspirations to serve God through the observance and study of the Torah, which was at the same time the localization of all people's aspirations to serve God through the full actualization of human powers -- and only in the absence of the Temple could the necessary reboot be undertaken.