Persons aged 16-25 will be assessed on their own income only if they are deemed to be financially independent.
This means that if your daughter's income (her wages, disability allowance when it comes through and student grant if eligible) is equal to or greater than the single rate of SWA (currently €197.80 per week), she'll be means-tested in her own right rather than on your incomes.
If she does not meet the means test limits, she can appeal and, given the circumstances outlined, could be granted a card on "undue hardship" grounds - where her requirement for medical services would cause her financial and other difficulties without a medical card.