Words like "sky" and "money" have "ies" as a plural suffix (i.e. "skies" and "monies") but other words like "monkey" and "Emmy" do not ("monkeys" and "Emmys"). Is there a rule dictating the use of "ies"?
Monies is one of those words where an error crept in and is now accepted as an exception. The plural of money (and there IS a plural in legal and accounting) should be, and is still acceptable as, moneys, and then there isn't an exception.
The general rule - if the Y is preceded by a consonant, then it is replaced by ies. If the Y is preceded by a vowel, add s.
This also works for verbs (I carry - he carries, I play, he plays). Nice and consistent.