Sometimes cultures have catastrophic memory failures. Memory erasures. Disruptions. Ireland in the sixth century provides us with a good example of this. Something happened. Something huge. We can see it laid bare in the archaeology: a revolution. Fundamental shifts in the way that people lived, farmed, ate, organized themselves. People started to build ringforts and crannogs (settlements in the middle of lakes). Palisaded enclosures. But we don't really know what happened or why.

[...] Our sparse records from that period refer vaguely to 'the first mortality' in 545, 'a great mortality' in 549, a 'pestilence' in 554, and another 'great mortality' in 556. They list battles, the deaths of kings, the foundations of churches. Beneath their laconic testimony the changes on the ground - visible in the archaeology but invisible in the textual record - show the radical transformation of communities.

[...] The social changes of the sixth century are one of the many reasons why it is highly unlikely that many - or any - practices or beliefs from pre-Christian Ireland survived intact into the Christian era.

Elizabeth Boyle, 'Fierce Appetites'