May also be referring to the incapacitated child tax credit. It's worth €1,000 in 2005 and was €500 for 2002 - 2004.
You need to write to your tax office and request the tax credit backdated to the year of birth. You will need a letter from your GP/consultant confirming the disability.
Your letter should say something like this
Re: Your Name
Ref: Your PPS Number
Dear Sir/Madam,
I would like to claim the incapacitated child tax credit in respect of my daughter X. X was born on 1 January 2005 and has been diagnosed with Autism.
X’s PPS number is 1234567L.
I attach some certificates from Department of Education and from X’s GP (whatever you have).
I would appreciate if you would grant this tax credit in respect of X and backdate it to date of birth.
If there is any problem in this regard or you have any queries please do not hesitate to contact me.
Yours sincerely,
Best of luck