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The closest useful Irish chronological marker is an organic deposit exposed at Derryvree near Maguiresbridge, County Fermanagh. There, freshwater silts and fine sands lay between two tills: sediments left by an earlier ice advance below and a later one above. The intervening organic layer produced the legacy radiocarbon determination Birm-166, 30,500 ± 1,100 radiocarbon years BP. Published calibrations place it broadly at about 36,900–31,700 calibrated years BP. The modern label 31,559 BCE, approximately 33,509 calibrated years BP, falls within that wide range, but it cannot be assigned to the beginning, end or any particular season of the deposit.
Ireland was not yet uniformly buried beneath the last great ice sheet at this point. The British–Irish Ice Sheet would later expand across the island and beyond its present western shoreline, reaching its greatest Late Pleistocene extent tens of centuries afterwards. Work on the chronology of the ice sheet indicates a long build-up from broadly ice-free conditions in parts of Ireland at about 34,000 years ago to major expansion later in the glacial cycle. That broad reconstruction matters more than any claim that an ice margin stood at a precise place in 31,638 BCE.
The most useful Irish environmental evidence comes from Derryvree near Maguiresbridge in County Fermanagh. Road works in 1969 exposed freshwater silts and fine sands trapped between two layers of glacial till within a drumlin. The organic deposits produced a radiocarbon determination of 30,500 ± c.1,100 radiocarbon years before present. When calibrated, that determination has been expressed as a wide range of approximately 36.9–31.7 thousand calendar years before present. The modern date 31,699 BCE falls near the younger end of that range, but it cannot be narrowed to a season, generation or individual year.
Radiocarbon dates are not calendar dates in themselves. The concentration of radioactive carbon in the atmosphere has varied over time, and especially so during the Pleistocene. Dates must therefore be calibrated against internationally maintained curves, including IntCal20. At around 30,000 radiocarbon years ago the relationship between radiocarbon age and calendar age is particularly complex. A result may calibrate to a broad range, sometimes with several possible intervals. Stratigraphy—the physical order of layers—remains as important as the numerical determination.
The most illuminating evidence close to this broad horizon comes from Derryvree, near Maguiresbridge in County Fermanagh. Road works in 1969 cut through a drumlin and exposed fine sands and organic freshwater silts lying between two thick deposits of till. This sequence is unusually valuable. The lower till records an earlier phase of glaciation; the silts accumulated when water, plants and animals occupied an ice-free local basin; and the upper till shows that later ice overran and sealed the deposit. It is a rare surviving fragment of a landscape that subsequent glaciation mostly destroyed or buried.
For Ireland around this point, there are no names, communities, settlements, graves, tools in secure context, hearths or written traditions. There can therefore be no responsible account of rulers, diplomacy, warfare, law or political boundaries. Nor is there evidence for a permanent population. This is not a failure to look hard enough for a lost Irish kingdom: it reflects the immense antiquity of the period, subsequent glacial erosion, changing sea levels and the very small amount of surviving Palaeolithic evidence on the island.
Inter-till freshwater deposits near Maguiresbridge in County Fermanagh show cold open habitats with treeless vegetation and freshwater life. Castlepook Cave in north Cork preserves remains of reindeer, mammoth, giant deer, bear, wolf, fox, Arctic fox and spotted hyaena. Together these sites show that Ireland supported substantial animal life before the later maximum spread of ice.
At this horizon Ireland was entering a decisive phase in the growth of the last British–Irish Ice Sheet. That growth was neither simple nor simultaneous. Ice expanded and retreated in different regions at different times, responding to climate, sea level, topography and its own internal dynamics. Later parts of the glaciation covered or disturbed great areas of the island, eroding older surfaces and burying valuable deposits beneath till. Consequently, surviving evidence is patchy and local. It would be wrong to picture all Ireland as either a continuous ice sheet or an entirely open plain in 31,810 BCE.
Ireland was not yet at the peak of the last glaciation. The global Last Glacial Maximum came later, broadly between about 26,500 and 19,000 years ago, although the timing of maximum ice extent differed from one sector of the British–Irish Ice Sheet to another. Around 33,800 cal BP, the island lay in a long and unstable approach towards more extensive glaciation. Ice did not simply advance as a smooth, uniform blanket. Its growth, movement and retreat varied by region, while climate alternated between colder stadials and relatively milder interstadials.
