This constituency comprised the south-eastern part of County Dublin.
From 1885–1918, it was a strip along the coast south of the city of Dublin to the county boundary. The constituency was bounded by the city of Dublin to the north, North Dublin to the west, East Wicklow to the south and the sea to the east. It included Dalkey, Kingstown, Blackrock, Stillorgan, Glencullen.
In 1918–1922 South Dublin was the southernmost of three constituencies south of the city of Dublin. The constituency boundary was also pushed a little further west than that of its previous incarnation had been. The other two successor constituencies to the 1885–1918 South Dublin were Rathmines, south of the city of Dublin, and Pembroke to the north of South Dublin. The other surrounding constituencies were unchanged.
1885–1918: The barony of Dublin, that part of the barony of Rathdown not contained in the constituency of North Dublin, and that part of the barony of Uppercross within the parishes of Donnybrook, St. Catherine's and St. Peter's and the townland of Cherry Orchard in the parish of St. Nicholas Without.
1918–1922: That part of the rural district of Rathdown No. 1 not contained in the constituency of Pembroke and the urban districts of Blackrock, Dalkey, Killiney and Ballybrack, and Kingstown.
At the general elections of 1885 and 1886, the Irish Parliamentary Party candidate gained a majority of the votes cast. At the general elections of 1892, 1895, 1900, 1906 and January 1910, Unionist candidates gained a majority of the votes cast, although in 1900 the Unionist vote was split and the Irish Parliamentary Party candidate was elected. In January 1910, the Unionist majority fell to 66, and in December 1910, the Irish Parliamentary Party candidate was returned with a majority of 133. 'The unionists had held on to the ... seat with the help of loyal upper and middle-class Catholics. When the seat eventually fell to the nationalists in the second election of 1910 the successful candidate was William Cotton, a leading figure in the business community whose patriotism was broad enough to allow him to support motions for loyal addresses to the monarch at Dublin Corporation meetings ... many nationalists were suspicious of Cotton’s conservative views' At a by-election in July 1917, the Irish Parliamentary Party candidate was returned unopposed. Following a redrawing of boundaries, the seat was won by the Sinn Féin candidate at the general election of 1918.