Rathdown is the south-easternmost barony in County Dublin, Ireland. It gives its name to the administrative county of Dún Laoghaire–Rathdown. Before County Wicklow was shired in 1606, Rathdown extended further south: it was named after a medieval settlement which grew up around Rathdown Castle, at a site subsequently deserted and now in County Wicklow in the townlands of Rathdown North and South, north of Greystones. The Wicklow barony of Rathdown corresponds to the portion transferred to the new county; although both divisions were originally classed as "half baronies", in the nineteenth century the distinction between a barony and a half barony was obsolete.