According to Wikipedia:
http://en.wikipedia.org/wiki/Irish_coffee

"Although this beverage may be served without sugar,
doing so would prevent the cream from floating on top
of the coffee in the traditional manner."

The recipe given has only 1 teaspoon of sugar
in 150 ml of Irish coffee, and this seems hardly
enough to modify the properties of the liquid
at all. Certainly not enough to substantially
change it's specific gravity, osmotic pressure,
ionic strength, etc. So how does it affect
floating the cream on top? Is this just a
bartender's myth?

It seems to me this is a ripe field for
experimental chemistry, if it hasn't been
done already. Why do chemists study stuff
that we can't use -- like the solubility of
noble gases in organic solvents -- when there
are practical problems of everyday life for
which everybody can benefit from the research?