Most Social Welfare income is assessable for medical cards and the guidelines tend to be strictly applied, no matter how small the excess.
Remember that mortgage, rent and travel costs to work are allowable when arriving at your net assessable income.
If your sole income is a Social Welfare means-tested allowance (e.g. Jobseeker's allowance), a medical card is generally granted, even if the SW income exceeds the guidelines (this is because the medical card guidelines haven't increased in line with SW increases in recent years).
If you are refused a medical card, you have a right of appeal. If ongoing medical expenses cause undue hardship (e.g. inability to pay rent/mortgage, provide for family), a card may be granted.
There is also a GP visit card, the income guidelines are approx 25% higher than the medical card, so you may qualify for one instead.
And finally, persons over 70 get a medical card regardless of income.