I can't find my grandad birth certificate any where
https://civilrecords.irishgenealogy.ieMaiden surnames of mothers and dates of births are shown in results table for births registered during the years you think your grandfather was born.
A search for James Brady births registered 1916-1920 anywhere in Ireland produces 131 results. They include babies with other surnames if mother's maiden surname was Brady. Select the option to put results in order of relevance. That will place James Brady births on the list before those with other surnames.
As you're not certain where your granddad was born you may have to consider them all.
Either print them off or make a note of each James Brady birth, including date of birth, registration district and mother's maiden surname.
Then as he was, according to your aunt, a twin, search for other Brady births in each of those districts in the same year/s and with the same maiden surname. If you get any matches, check if date of birth was the same (or a day apart in case twins were born either side of midnight). There's an advanced search page where you can enter relevant information. Select "More search options".
This will only work if James was a twin and if his forename was included on the birth registration.
Someone else may be able to suggest a quicker method of filtering results.
