If you instead take the late-Latin (in use c. 1250 CE in Britain) millenium or the English word centennial as the model, then quattourdecimcentennial(is) is probably more correct, using the cardinal number fourteen.You could even make an argument for quaternidenicentennial(is), using the distributive. All of these should make a certain amount of sense to an English speaker familiar with Latin.
If you want something that a native speaker (or scholar of the language) might more readily write, millensimus quadringentensimus is probably close. Livy has mille et quadringentis for the cardinal 1400 (Ad Urbe Condita 26.50), and I'd assume mille(n)simus (et) quadringente(n)simus to be the ordinal equivalent (those 'n's are dropped pretty regularly, and the 'et' is entirely optional.) It would decline as a regular first/second declension adjective on the model of bonus, -a, -um; so 1400th year (nominative) would be millenimus quadringentesimus annus. It's a little trickier if you want to refer to a specific event which has recurred once every year for 1400 years, but you'd probably want to use anniversarius (yearly) in some form: eg, millesima quadringentesima anniversaria lupercalia, the 1400th annual Lupercalia. I really don't know enough about ecclesiastical Latin to say whether there were other conventions for writing numerals by the 7th century, but this would at least make sense when read. You are certainly correct that (written) Latin in Ireland was almost dialectally different—Hisperic Latin is a very strange creature, and I know nothing about that, either, except that the Altus Prosator is often given as the prime example.