Zeuss: The Grammatica Celtica: The first to make extensive use of the glosses for these purposes was Johann Kaspar Zeuss, a Bavarian; born 1806; died 1856. He had a great talent for languages, and began the study of the Celtic dialects about 1840. Thenceforward he laboured incessantly, visiting the libraries of Saint Gall, Wurzburg, Milan, Carlsrühe, Cambrai, and several other cities, in all of which there are manuscript books with glosses in the Celtic dialects; and he copied everything that suited his purpose. He found the Irish glosses by far the most ancient, extensive, and important of all. Most of them belonged to the eighth century; some few to the beginning of the ninth. At the end of thirteen years he produced the great work of his life, Grammatica Celtica, a complete grammar of the four ancient Celtic dialects: published 1853. It is a closely printed book of over 1000 pages, and it is all written in Latin, except of course the Celtic examples and quotations. Each of the four dialects is treated of separately. In this work he proves that the Celtic people of the British Islands are the same with the Celta3 of the Continent; and that Celtic is one of the branches of the Aryan or Indo-European languages, abreast with Latin, Greek, the Teutonic languages, Sanscrit, &c.
  Zeuss was the founder of Celtic philology. The Grammatica Celtica was a revelation to scholars wholly unexpected, and it gave an impetus to the study, which has been rather increasing than diminishing since his time. He made it plain that a knowledge of the Celtic languages is necessary in order to unravel the early history of the peoples of Western Europe. It is now quite a common thing to find scholars from continental countries visiting and residing for a time in Ireland to learn the Irish language. Since the time of Zeuss many scholarly works have been written on Celtic philology: but the Grammatica Celtica still stands at the head of all.

Alfred Webb’s account of John O’Donovan’s review of Grammatica Celtica in The Ulster Journal of Archaeology (1852-53).

‘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.’