“Blown to smithereens” is such a great, colorful phrase. Almost everyone knows exactly what you mean, without being able to define what exactly a smithereen is. What the heck are they?
Exactly what you think they are. A smithereen is simply a very tiny piece or fragment of something, and derives from an Irish word for the same. The Oxford English Dictionary and other sources agree that smithereen entered English sometime in the early 1800s from the Irish Gaelic smidirín (“small fragment”), which itself is just the Gaelic smiodar ("fragment") with a diminutive suffix attached.