Medieval Irish philology studies the language and manuscript tradition of medieval Irish texts, including early Christian and mythological literature, establishing what a text actually says and how its wording has been transmitted across its surviving copies. The Bavarian scholar Johann Kaspar Zeuss founded the discipline's rigorous grammatical basis in his Grammatica Celtica of 1853, built specifically from Old Irish glosses, marginal notes explaining a Latin manuscript's grammar, that he traced through continental monastic libraries at Wurzburg and elsewhere, evidence no earlier scholar had systematically read as a source for the Irish language itself.
A medieval Irish saga or saint's life is rarely preserved in a single stable form, and Zeuss's gloss based grammar is what let later philologists establish the actual Old Irish wording of the earliest layers of that literature rather than reading a much later manuscript copy as though it faithfully preserved the original text unchanged. The atlas relies on the discipline wherever a claim about early Irish Christian or pre-Christian religious literature depends on the text's precise medieval Irish wording rather than a later retelling.