before the transition to Old Irish. Consonant lenition and palatalisation may already have existed in an allophonic form, i.e., it was not phonologically contrastive yet. Transition to Old Irish Old Irish, written from the 6th century onward, has most of the distinctive characteristics of Irish, including "broad" and "slender" consonants, initial mutations, some loss of inflectional endings, but not of case marking, and consonant clusters created by the loss of unstressed syllables, along with a number of significant vowel and consonant changes, including the presence of the letter p, reimported into the language via loanwords and names. As an example, a 5th-century king of Leinster, whose name is recorded in Old Irish king-lists and annals as Mac Caírthinn Uí Enechglaiss, is memorialised on an ogham stone near where he died. This gives the late Primitive Irish version of his name (in the genitive case), as . Similarly, the Corcu Duibne, a people of County Kerry known from Old Irish sources, are memorialised on a number of stones in their territory as . Old Irish filed, "poet (gen.)", appears in ogham as . In each case the development of Primitive to Old Irish shows the loss of unstressed syllables and certain consonant changes. These changes, traced by historical linguistics, are not unusual in the development of languages but appear to have taken place unusually quickly in Irish. According to one theory given by John T. Koch, these changes coincide with the conversion to Christianity and the introduction of Latin learning. Many languages have various registers or levels of formality, the most formal of which, usually that of learning and religion, changes slowly while the most informal registers change much more quickly, but in most cases are prevented from developing into mutually unintelligible dialects by the existence of the more formal register. Koch argues that in pre-Christian Ireland the most formal register of the language would have been that used by the learned and religious strata, the druids, for their ceremonies and teaching. After the conversion to Christianity the druids lost their influence, and formal Primitive Irish was replaced by the then Irish of the common people and Latin, the language of the new learned group, the Christian monks. The vernacular forms of Irish, i.e. the ordinary Irish spoken by the common people (formerly 'hidden' by the conservative influence of the formal register) came to the surface, giving the impression of having