Make Boulder out of good British stone, then I will: on St. George’s Day.

The Blarney Stone (Irish: Cloch na Blarnan) is a block of Carboniferous limestone built into the battlements of Blarney Castle, Blarney, about 8 kilometres (5 mi) from Cork, Ireland. According to legend, kissing the stone endows the kisser with the gift of the gab (great eloquence or skill at flattery). The stone was set into a tower of the castle in 1446. The castle is a popular tourist site in Ireland, attracting visitors from all over the world to kiss the stone and tour the castle and its gardens.  
The word blarney has come to mean “clever, flattering, or coaxing talk”. Irish politician John O’Connor Power defined it this way: “Blarney is something more than mere flattery. It is flattery sweetened by humour and flavoured by wit. Those who mix with Irish folk have many examples of it in their everyday experience.”
 
That is quoted from wikipedia
 
All I know is it is a stone in a very old castle wall that is falling apart that people go to kiss.  
Some tourists think it will help them communicate with the locals better… Ye Wah Bi? Good luck with that. Natives can hardly understand each other with all the thick and heavy accents.
 
I kissed it, and head butted it when my dad lifted me back up. No gift of the gab, I still hardly talk at all since before and after.
 
I guess it is just something you can gab on about doing for some people.

Why exactly does one need to kiss the Blarney Stone? Where’d it originate from?

This would be her favorite part of the holiday.