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Radiocarbon measurements do not translate directly into calendar years. Carbon-14 levels in the atmosphere changed through time, so laboratory ages must be calibrated against independently dated records. At around 30,000 BCE, the calibration curve is less precise than it is for recent millennia and can produce wide calendar intervals. Furthermore, a date on an animal bone establishes when the animal died, not necessarily when a particular mark was made on it, when sediment moved, or when people may have visited a cave. These distinctions matter greatly for prehistoric Ireland.
At this depth of time, dating requires care. Radiocarbon years before present are counted from AD 1950 and are not identical to calendar years. Their conversion depends upon calibration curves, and the resulting ranges can be wide. The famous organic sediment at Derryvree near Maguiresbridge, County Fermanagh, produced the conventional radiocarbon determination BIRM-166: 30,500 years BP, with a substantial uncertainty. When calibrated, it belongs broadly to a span of roughly 36,900–31,700 calibrated years BP, or approximately 34,950–29,750 BCE. Thus 30,332 BCE sits within the wide chronological horizon relevant to the deposit, but it cannot be assigned to a particular season, pool, animal or human action represented there.
The key Derryvree radiocarbon determination, conventionally known as BIRM-166, was obtained from the organic freshwater deposits and reported as 30,500 radiocarbon years before present, with a substantial error range. When calibrated, it falls broadly within approximately 36,900–31,700 calendar years before present. The modern date-label 30,456 BCE lies within that very wide calibrated span, but it must not be mistaken for the age of every seed, moss fragment or insect in the bed. Nor can it identify a particular season in which the basin was open water. The deposit accumulated over time; its age is an estimate with a range, not a historical timestamp.
