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.