Письменные памятники ирландского языка известны с 4 века нашей эры. Период с 4 по 6 век исследователи признают переломным: язык за очень короткое время трансформировался из Primitive Irish (архаичного индоевропейского типа, подобного другим древним и.-е. языкам) в «кельтский модерн» (хотя и называемый Old Irish) с мутациями согласных и прочей кельтской спецификой.
Мне понравилось объяснение того, каким образом это могло случиться так быстро, из английской Википедии (статья
Primitive Irish):
Цитата:
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 MAQI CAIRATINI AVI INEQUAGLAS. 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 DOVINIAS. Old Irish filed, "poet (gen.)", appears in ogham as VELITAS. 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. All 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 class, 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 Upper Class Irish of the nobility and Latin, the language of the new learned class, the Christian monks. The vernacular forms of Irish, i.e. the ordinary Irish spoken by the upper classes (formerly 'hidden' by the conservative influence of the formal register) came to the surface, giving the impression of having changed rapidly; a new written standard, Old Irish, established itself.
Для частоты употребления правильных и неправильных языковых конструкций нет единого распределения, поэтому разговоры о "центрах" и "дисперсиии" и т.п. - полная бессмыслица! 
