I looked into that a little while back, the CSO changes misadventure findings to suicide where necessary for statistical purposes. They know what coroners are doing and try to weight the figures accordingly. So lets say 400 suicides are recorded by coroners, there could be 500 recorded by the CSO based on qualitative review of cases.
So on that side, it's not under reported too much, however there are a lot of accidents, particularly single vehicle car accidents that may be suicides, these don't get recorded as such
I suppose the figures can never be accurate. However the rate is alarming and something like the RSA should be set up to tackle it head on.