In contrast to modern linguistic etymology, which studies the origin of words (combining expression and meaning), their interrelationship, and their historical changes, medieval etymology is ontological insofar as it assumes that the relationship between the signifier (word, name) and the signified (thing, person) is not arbitrary but that the investigation of the former will throw light on the nature of the latter. ("Etymology" < etymologia, is from Greek etymon "the true, original thing" + -logia = "the science of origin"). In modern times, medieval etymology has been much ridiculed because of its "unscientific" approach, especially its way of using or adapting morphologically similar and semantically suitable words, its method of dividing words into (sometimes dual language) components (Early Irish belra n-etarscartha "the science, literally language, of separation"), and the absence of the postulate of uniqueness. Medieval etymology is theoretically well founded.
It has been claimed that the Bible with its numerous etymologies and etymological origin tales—see, for instance, the double explanation of the names of the sons of Jacob or the origin tale of Passover—was the model for the medieval Irish scholars. However, the stimulus for systematic etymological research and application by Irish scholars from the seventh century onward came from Isidore (Bishop of Seville, f 636). His Etymologiae (also called Origines), an encyclopedic collection of heterogeneous materials arranged according to subjects, was at the same time their methodological guide and their practical model. Isidore’s philosophy is briefly: Omnis enim rei inspectio etymo-logia cognita planior est. (The investigation of every thing is clearer once the etymology is known.) (Etym. I 19,2; see also I 7,1.)
In Irish texts etymologies are found as part of interlinear and marginal glosses or they are integrated into the text as, for instance, the explanations of technical terms at the beginning of the legal text Crfth Gablach. Etymology is either implicit or identified as such. There are general collections (alphabetically arranged like book X "De vocabulis" of the Etymologiae) such as Cormac’s Glossary and specialized collections such as the legal O’Davoren’s Glossary, which includes valuable quotations, or the more elaborately explained names (epithets) of famous persons in Coir Anmann (The Correctness of Names). The self-contained etymological origin tales of the names of places, in prose and verse versions, are called Dinnsenchas (The Lore of Famous Places). In the prehistoric ("synthetic") part of the Irish history book An Lebar Gabala (The Book of Invasions; Invasion Myth) whole sections originate in this type of etymology—see, for instance, the explanation of Scotti (i.e., the Irish) from Scythi (otherwise also from Pharaoh’s daughter Scotta).
In double-barreled place names the element after generics like dun (fort) or sliab (mountain) was usually interpreted as the name of a person. Thus in the Ulster epic Tain Bo Cuailnge at a place called Ath Buide (presumably, The Yellow Ford) Cu Chulainn killed an adversary by the name of Buide; therefore the ford was called Ath Buide (Buide’s Ford), as is explicitly stated. In Betha Senain (The Life of St. Senain) the name of his island, Inis Chathaig (Scattery Island), is elaborately explained through the presence and activities of a monster called Cathach, which he expelled.
