The dynamics and feeding biology of a population of Marthasterias glacialis (Echinodermata: Asteroidea) was examined over a two-year period from 2000–2002 at Lough Hyne Marine Nature Reserve, Co. Cork, Ireland. A multivariate approach was used and both multiple factors and multiple interactions between factors were found to influence population structure. These included time of year, site, individual sea-star size, depth, and predator density. Individuals belonging to the smallest size-classes (0–50 mm and 51–100 mm) were most abundant amongst boulders in shallow water (0–1 m), while larger individuals were primarily found in water below 1 m in depth on finer grade substratum and shell debris. Dietary composition was also found to differ with depth; sea-stars in the immediate subtidal had an opportunistic diet, and fed on a variety of taxa, whilst those M. glacialis from 1–6 m were more selective and restrictive, feeding chiefly on bivalve prey. We propose that spatial partitioning of different size-classes and a generalist feeding strategy may account for the success of M. glacialis at Lough Hyne.