Land and Property Services Northern Ireland (into which OSNI was folded a few years back) is not part of the UK government's OpenData initiative, so while there are shapefiles available for administrative geographies such as parliamentary constituencies, they operate on the basis of a sizeable annual licence fee.
In addition, the people running LPS NI decided on April 1 to announce:
There is no word as to when the new website will become available.
What's available at the moment, is the set of NISRA's census geography files at:
http://www.nisra.gov.uk/geography/SOA_.htmSOAs can be nested into wards, which in turn can be nested into parliamentary constituencies.
Alternatively, if you feel like doing some web service querying, and conversion of JSON into shapefiles, there is:
http://webservices.spatialni.gov.uk/arcgis/rest/services/LPS
