Abstract During the early to mid-Ordovician, marine life experienced an unprecedented rise in diversity at the species, genus and family levels that firmly installed the suspension-feeding benthos as the main component of the Palaeozoic fauna. The earlier Ordovician was characterized by a wide dispersal of the continents together with a high frequency of microcontinents and volcanic arcs. Magmatic and tectonic activity was intense, climates were warm and sea levels were high. Central to the entire diversification is the role of gamma (inter-provincial) diversity and by implication the spread of the continents and frequency of island arcs and microcontinents. A disparate group of continental fragments and island arcs loosely assigned to the Celtic province contained distinctive shelly faunas that formed a testable biogeographical unit. The Celtic faunas are characterized by a large number of endemic brachiopod taxa, some cosmopolitan forms, and taxa at the beginning or end of their stratigraphical ranges. The associated trilobite faunas are composed largely of wide-ranging genera, a few genera at the start of their stratigraphical range and some recently evolved taxa extending their geographical range. The Celtic province helped provide a burst of gamma diversity during the early stages of the Ordovician Radiation whereas the timing and position of the archipelagos associated with the Celtic province may have provided a mechanism for the diachroneity associated with the diversification.