The Disability Act (Government of Ireland, 2005) defines disability as:
“A substantial restriction in the capacity of the person to carry on a profession, business or occupation in the Irish State or to participate in social or cultural life in the Irish State by reason of an
enduring physical, sensory, mental health or intellectual impairment.”
Basically disability is pretty well permanent in nature (hence not needing to keep proving it - to be utterly crass about it, missing legs don't suddenly regenerate - for example.) Disability generally makes it much harder to obtain work in the first place. The allowances are not based on contributions and is means tested. Generally people who have difficulty obtaining work do not have the means to support themselves. More information
here
Invalidity is not necessarily permanent. It is a long term illness. Hence the reason why it needs to be proven by assessment and the reason why being deemed fit for work changes your entitlement (if still unemployed you would be considered a jobseeker in that case). The allowances are based on your contributions. More information
here
You aren't being "penalised". If you hadn't worked or hadn't enough contributions then you would not be entitled to claim invalidity pension.