This paper discusses how the production rate of historical and archaeological data might contain unique information about past societies. The case study is the frequency of entries in the Annals of Ulster, a primary early medieval source from Ireland, which was compared to the frequency of archaeological material from early medieval Ireland. The two datasets were found to contain similar trends, namely a rapid increase in activity in the 7th Century, followed by a decline in the Early 9th Century, low levels of activity in the 10th Century, until recovery in the Late 10th / Early 11th Centuries. This overall pattern of activity had not been noticed before. Turning to the archaeological record of Britain, although there are certain similarities between Ireland and Scotland especially in the early part of the period, we find that the 9th and 10th centuries there were a stable period, and thus contrast with Ireland. We argue that environmental pressures are unlikely to be driving the signal, and instead various socio-cultural factors in the past coalesced in Ireland, leading to circumstances powerful enough to attenuate the enduring evidence for human activity, but expressing themselves silently, perhaps even in a way that was not immediately obvious to those witnessing them in the past. The simplest explanation, we contend, is that population levels fell throughout the period. This finding offers insight into the relationship between long-term change and the primary production of history, and supports the idea that the quantity of certain historical data can contain information about past realities.