My DS has a pre school assistant, he attends Enable Ireland and they gave me the address to send the letter to and told me what I should write, there wasn't an application form as far as I remember. Get in contact with the early intervention services that your child attends, they would be more familiar with this. My public health nurse was not involved at all, any letter of referral will do. The pre school teacher wrote a letter outlining DS' difficulties and he successfully got one, she started last week.
Best of luck with it.