I went to the census of 1911 and found William Breen aged 11 in the Artane Industrial School. I looked for his brother but couldn't find him there. There is a Christopher Breen but the information fizzeled out as you go down through the records. It's like whoever was writing them just got fed up with putting in the information about each child. I'm surprised the enumerator accepted these forms in that state!!!
It seems that William would have been born about 1900. You will nedd his parents names to order his birth certificate.
Because you are a direct decendant of William, I believe that you have a right to view his records. This would give you his parents names (without which you can't go back any further). You may also find out more about his siblings.
I think it is worth contacting the Department of Education and find out what information you can get. I know you will probably need your grandfathers death certificate, and proof that you are related to him. This is to safeguard peoples records. You will need to make an application on behalf of a deceased former resident.
Ring to find out more (like many people are doing now)
10 8892240 01 8892385 01 8896444
I wish you well in your search,