You'd probably have to ask you're guidance councillor, but as far as I'm aware you have to have the said disability assessed by a professinal (so like a educational psychologist or a consultant or a physiatrist)in the past 2/3 years and then you have to fill out a form dissuading how it negatively affects your school life. You'd have to look up their website (accesscollege.ie) for the specifics of the disabilities and what qualifies you etc, and only some colleges use the DARE scheme some have their own one.
Yes, it's judged on how many days absent from school because of said disability (it has to negatively impact your school life). You also have to have documentation from your doctor proving you actually have the disability and this can take ages to sort out so I'd recommend you start asap. There's no harm in applying even if you're not 100 % sure you'll get it.