Changes in the distributional ranges of marine indicator species have been used to support hypotheses of climate change on NE Atlantic shores. One such indicator species, Osilinus lineatus (da Costa, 1778) (a trochid gastropod), recently colonized the Lough Hyne Marine Reserve in SW Ireland and spread around the lough shores in the last two decades. The incursion and local spread of O. lineatus was documented during annual intertidal community surveys of the lough from 1994 to 2010. This species is presently an abundant member of the lough's biota. Although the tidal rapids connecting the lough to the open ocean may have acted as a partial barrier to incursion, substantial populations of the trochid have been noted within the lough from 2003 to the present. Climate change may have enabled this primarily Lusitanien species to penetrate and flourish in the lough. However, based on environmental variables measured and other species surveyed, changes in the semi-enclosed lough may be more complex.