How are A Type Credits useful

I have been on disabilty allowance for two years, only had 18 AI Contributions in the 2009. I understood that I was getting an A type credit each week on allowance. However when I applied for Invalidity Pension , I was told that I would need another 8 paid contributions, before I would receive A type credits
And that I would need 48 in 2012, and apply in 2013

This seems madness, if you are unable to work under serious medical conditions, that there is not a medical assessment, and it does not mater that you are short contributions at the time of illness

What was your employment/prsi record like before 2009?

In general you do get credits if you are on an allowance scheme like Disability Allowance, but this is not the case if you have had a gap of 2 years or more in your insurance record. If you have a gap of 2 years in your record then you are not eligible for credits until you have made another 26 paid contributions, which would explain why they say you need another 8 on top of the 18 that you have already.