Swansea Cork ferries are not operating any services throughout 2007. Their website states that they were unable to find a suitable vessel but they hope that services will resume in 2008.
According to the Ireland tourism site
http://www.discoverireland.com/gb/ireland-plan -your-visit/getting-there/by-sea/
the nearest alternatives are Pembroke to Rosslare with Irish Ferries
http://www.irishferries.com/
or Fishguard to Rosslare with Stena
http://www.stenaline.co.uk/
Otherwise, you might wish to consider flying from Cardiff to Cork, with Aer Arann:
http://www.aerarann.ie/
Chris