Dear Searcher, The first thing to do is to try to identify the baptism of Elizabeth, Margaret or Adelaide Kiernan
However, you will need to know what religion the girs were likely to have been baptised into.
If they were baptised Catholic, you will find a number of websites online with some parish registers,
As your grandmother was born in Dublin, its important to be aware that these appear on a number of different websites.
Irishgenealogy.ie has some, rootsireland.ie has some
But for the complete collection, try findmypast.ie
If the Kiernans were Protestant, then again, its important to be aware that many of Dublin COI records were destroyed.What survives are currently being digitised and when these go online it will open up more knowledge about our ancestors.
Good luck with your search!