I applied for it in Jan 2009 and got it finally in april so HSE paid for Jan/feb/mar as arrears, a nice windfall.
However I have been on JB since Aug 2008 and they wouldn't cough up for those 5 months. I said to the CWO that I hadn't heard of it but HSE didn't want to know. I was really annoyed at myself for losing out on thousands of euros but at least I'm getting half the mortgage paid now so I'm delighted and very grateful.
For social welfare schemes in general, not being aware of the scheme/entitlement is not sufficient reason for backdating of any claims. In fact, many of the forms state this, AFAIK.