Do you happen to know something about the coinage in Ireland in that period? Also, searching for the survived original sources in that entry, they are either first half 400, written outside of Ireland, then some written later than 600 AD? Maybe I missed something?
https://en.wikipedia.org/wiki/Paganism
"The notion of paganism, as it is generally understood today, was created by the early Christian Church. It was a label that Christians applied to others, one of the antitheses that were central to the process of Christian self-definition. As such, throughout history it was generally used in a derogatory sense. - Owen Davies, Paganism: A Very Short Introduction, 2011"
"The term pagan is not attested in the English language until the 17th century.[24] In addition to infidel and heretic, it was used as one of several pejorative Christian counterparts to gentile as used in Judaism and to kafir ('unbeliever') and mushrik ('idolater') as in Islam.[25]"
Interestingly, Ireland during the 400's to 600's was something of a bastion of Catholicism and 'western/classical' thought in their monasteries. Even though Britain was very pagan and 'dark', Ireland remained 'enlightened' at this time (though the populace was still very 'dark' and pagan in Ireland).
https://en.wikipedia.org/wiki/History_of_Ireland_(400%E2%80%...
Under this regional definition of 'dark ages', even today they occur. Lost tribes in the Amazon, the populace of North Korea, very rural towns in Alaska, etc. all can be considered to be in 'dark' ages to some degree or another.