According to Matheson's Surnames in Ireland (1909) the name seems to be clustered around Antrim in Ulster with 18 events and just one event in Leinster.
The mormon library has a microfiche of Rotunda Hospital births which may (or may not) give you some extra info. I used it a few years ago successfully ...
Have you checked the 1911 census at http://www.census.nationalarchives.ie
There are quite a few Semples in Antrim but only a handful in Dublin ... you didnt mention the mothers name so i could narrow it down ...try and find her in 1911 for a start ... assuming that she was in ireland that year and the correct name was used for registration ;-)
Good luck ...