I was in touch with a Michelle Kelly from PRONI in 2009 telling me that Western Trust transferred all their records to them. Michelle also stated "we do have a large collection for Gransha which was transferred to us 3-4 years ago but there are a few gaps. It is most likely that the records that are missing do not exist any longer".
She did however did a detailed search of the relevant archives and could confirm that she found my great uncle, a Samuel McCandless and that his registration number was 4157. He was listed in the 1901 census but died in Gransha in 1907 aged 33. Michelle could not find his case notes, admission details, committal papers but I managed to get his death cert.
The poor man was in there for 6 years and 6 years of paper work has disappeared. Very frustrating.