The common denominator between “sardonic” and “sardines” has often been thought to be the island of Sardinia.
“Sardonic” comes from the Homeric sardanios, which means “bitter and scornful.” But Sardonios – please note the “o” instead of the “a” – means Sardinian, and the Greeks believed that eating a plant called sardonion – “the plant from Sardinia” – caused facial convulsions resembling those of sardonic laughter, followed by death. So they thought “sardanios” should properly be “sardonios.”
The word “sardine” seems to come from the Greek sardine. Scholars used to assume that its origin was Sardo, the Greek name of the island of Sardinia, near which the fish was “probably” caught and exported. “Probably” is the word used in the text consulted, with the important qualification that it was hardly probable that the Greeks obtained fish from so far as early as the time of Aristotle, who used the word in a passage quoted by Athenaios.
Therefore, as of today, it has to be admitted that scholars don’t know where the words “sardines” and “sardonic” come from. There may in fact be nothing intrinsically sardonic or sardine-loving about Sardinians.
But they do know that the phrase “packed like sardines” was first recorded in 1911.
Eric Koch’s book, The Weimar Triangle, is available at Indigo-Chapters and in your local bookstore. 
In fact, the association of sardines with Sardinia may be a red herring.
Toujours le mot juste.
The connection definitely sounds fishy to me. It needs to be taken with a pinch of salt, and perhaps some olive oil.
I am surprised you did not refer me to Sardanapalus, a play by Byron.