(1) Where a claimant proves that he or she has living at any time during a year of assessment any child who -
(a) is under the age of 18 years and is permanently incapacitated by reason of mental or physical infirmity, or
(b) if over the age of 18 years at the commencement of the year, is permanently incapacitated by reason of mental or physical infirmity from maintaining himself or herself and had become so permanently incapacitated before he or she had attained the age of 21 years or had become so permanently incapacitated after attaining the age of 21 years but while he or she had been in receipt of full-time instruction at any university, college, school or other educational establishment,
the claimant shall, subject to this section, be entitled in respect of each such child to a tax credit (to be known as the 'incapacitated child tax credit') of €3,500.
(a) A child under the age of 18 years shall be regarded as permanently incapacitated by reason of mental or physical infirm
…