How many people speak Irish in Dublin?

Not Gaelic. Irish. 

I did know a couple of people from Dublin who spoke Irish at home, but not many. Most native Irish speakers are concentrated along the west coast and on the islands. 

I think what you mean is Gaelic language. Most Irish can't speak Gaelic.

Very few.  Irish speakers are generally on the west coast.

Too few                                     

Wikipedia article:

https://en.wikipedia.org/wiki/Irish_language