Just found this post at the bottom of another thread.
The 1901 census will eventually be put online (National Archive, Dublin website) but not for a while yet. In the meantime, 1901 census is available at PRONI and possibly Ballymena Library (depending on where in Belfast they lived). As it is not yet indexed you'll have to have a rough idea of an area to start searching and just go through the roll, or rolls, page by page which is quite time-consuming.