Just in case anyone else is looking for Robertville in 1901 Census I have had clarification of the position and numbering with huge thanks to Jim Curtis and the Rathmines Historical Society. See below:
There are now thirty houses at the Terenure end of Harold's Cross Road, numbered 291-349, and these were originally in two separate terraces, Abercorn Terrace built around 1868, and Avondale Terrace (with bay windows) built around 1885. The small gap between these two terraces was filled-in around 1890 with two houses, called 1 and 2 Robert-ville. Initially, this locality was in the Rathmines Township, and after 1930 was controlled by Dublin Corporation, at which stage the two Robertville houses were listed as 321 and 323 Harold's Cross Road.
In the 1901 Census, this area was listed as "Harolds Cross Road (part of)", and No 1 Robertville was listed as House No 28, and No 2 Robertville was listed as House No 29. in the 1911 Census, No 1 and 2 Robertville are specifically listed.
And so it turns out my GGF William Smyth was living there in 1916. Cant believe I missed that as an option. Many thanks to all for your help particularly Dathai.... It was much appreciated.