I went to IreAtlas at: http://www.thecore.com/cgi-bin/ire-srch and entered Dublin in the County box, Rathdown in the Poor Law Union box, and got a bunch of townlands, among which is Brenanstown. Brenanstown is shown as a townland of 327 acres, in the County of Dublin, Barony of Rathdown, Civil Parish of Tully, Poor Law Union of Rathdown. Maybe this will help.