If this William lived to adulthood and stayed in Ireland then the civil BMD records on IrishGenealogy would be the place to start - marriages are often best as they give more details which you can use to compare with the known details. e.g. father's name and occupation, residence of the groom, and sometimes names of witnesses.
Do you have occupation details for the father and any other addresses for the family, and details on the other two sons, did they both stay and marry in Ireland - was William a witness at either of their marriages, were any of these brothers still in Ireland for the 1901 or 1911 census returns ?
Most of the Church Catholic and Church of Ireland records for the City area are also on IrishGenealogy website and have been transcribed - so a little easier to search for marriages than the civil records - I checked these and did not see a good match for a marriage of your William.
The difficulty with tracing forward from a baptism at this time without additional clues is that there are some possible complications that are difficult to follow up on - William could have died before the start of civil registration in 1864, he could have moved outside Dublin city or emigrated...