There are transcriptions on familysearch.org. If you have not already, I would suggest a reverse search, which may help you find siblings. I did that with my McGarrity family, once I knew the names of my GGM's parents.
Civil Registration commenced for Protestants about 1845; for Catholics about 1864.
Have you found a death record for William James Robinson? Depending on when and where he died, you may find his mother's maiden name in the death record.
There are efforts underway in Ireland to digitize the records, so they may be available online in a year or so. http://www.irishtimes.com/newspaper/finance/2012/0924/122432...
Rootsireland.ie has record transcriptions - pay per view
EmeraldAncestors.com offers monthly subscriptions.
There are helpful volunteers on http://cotyroneireland.com/