This paper presents a comprehensive overview of the Moira Mounds in the Porcupine Seabight, combining a review of currently published material with new data and interpretations. The Moira Mounds are circular to ovoid-shaped individual or clustered coral banks (referred to as "scleractinian spaced cluster macroreefs") located in the Belgica (carbonate) mound province, Porcupine Seabight, west of Ireland. They are up to 10 m high and 20 to 35 m in diameter. For the first time, a map showing the complete extent and location of the Moira Mounds is published, revealing 256 examples grouped into four areas mainly between 800 and 1100 m water depth. ROV groundtruthing revealed the importance of bedload sand transport in Moira Mound formation and development processes through sediment entrapment by coral frameworks (Lophelia pertusa and Madrepora oculata). New estimated growth rates for the Moira Mounds are comparable to those measured for much larger Irish coral carbonate mounds but an order of magnitude lower than rates from comparably sized reefs offshore Norway. As well as the framework corals. 26 macrobenthic invertebrate species were identified from a single box core that were similar to those communities found in adjacent off-mound habitats. It is proposed that the Moira Mounds are an example of the early stage growth phase of nearby giant coral carbonate mounds and represent a westward extension of the zone of active cold-water coral mound growth and therefore a deepening of the zone of cold-water coral growth over recent climate cycles. (C) 2010 Elsevier B.V. All rights reserved.