John O’Donovan contributed an analysis of the book to the Ulster Journal of Archaeology for 1859. He says: “The Grammatica Celtica has the name of being exceedingly hard to be understood. And so it is without a doubt ... We must recognize in the Grammatica Celtica purely a triumph of comparative philology ... He has succeeded in giving for the first time a wonderful analysis of the Celtic - ... of that original form of the language where all the modern dialects of it find their point of coincidence.” O’Donovan also says: “It contains proofs of the purely Japetic origin of the Celts. It demonstrates the following facts: (1) That the Irish and Welsh languages are one in their origin; that their divergence, so far from being primeval, began only a few centuries before the Roman period; that the difference between them was very small when Caesar landed in Britain — so small, that an old Hibernian most likely was still understood there; and that both nations, Irish and British, were identical with the Celtae of the Continent — namely, those of Gaul, Spain, Lombardy, and the Alpine countries. This is, in fact, asserting the internal unity of the Celtic family. (2) That this Celtic tongue is, in the full and complete sense of the term, one of the great Indo-European branches of human speech ... There must now be an end to all attempts at assimilating either Hebrew, Phenician, Egyptian, Basque, or any other language which is not Indo-European, with any dialect of the Celtic. The consequence further is, that, as far as language gives evidence, we must consider the inhabitants of these islands strictly as brethren of those other five European families constituting that vast and ancient pastoral race who spread themselves in their nomadic migrations, till in the west they occupied Gaul, and crossed over to Britain, and to Ireland, the last boundary of the old world... The Irish nation has had no nobler gift bestowed upon them by any Continental author for centuries back than the work which he has written on their language.”

Dr. Reeves adds: “Zeuss was the greatest benefactor that Irish literature can record in its list.” Some years after the publication of this work, Zeuss is said to have expressed some disappointment at the apparent indifference with which it was received. But he was little aware what a revolution was being effected in opinion, and what deep root it was taking in the minds of all Celtic philologists who were susceptible of good impressions. Zeuss was a tall, well-made, rather spare man, with black hair and moustache, giving one more the impression of a Slavonian or a Greek than of a German.