Steven Pinker gave us a nice exposition on irregular plurals in English in his famous The Language Instinct. It was fascinating how a leaf can fall but only leaves can accumulate in number a slightly irregular plural (final consonant voicing as well as the s). More dramatic was highly irregular plurals like tooth~teeth. What was dramatic? The way the irregular plural was replaced by a regular in compounds; but not all compounds. Where the lexical item had a real-life referent, as in maple leaves falling from the tree, the irregularity stayed. But if the lexical item referred to something other than its real-life referent, as in The Toronto Maple Leafs, where the maple leaf was now the name of a hockey team and a member was called “a Toronto Maple Leaf”, the plural was regularized. Saber-tooth tigers in large numbers were not referred to as *saber-teeth but saber-tooths.
How did this happen? That was the point of the unfolding of this unconscious process in the minds of native English-speakers: the plural formant, s, was stored in one place in the brain where grammatical items are stored and the lexical item stored in another; thus s and cup were joined together coming from their separate storage areas into cups. An irregular plural, OTOH, was stored as a lexical item, leaves, teeth, in the same way leaf and tooth were stored. This was shown by the fact that in word compounds where the first element is the object of a verbal second element, e.g. rat-eater, the word rat doesn’t get pluralized even though the meaning refers to more than one rat, rats in the aggregate. The compound is formed before the s can be put on. But with irregular plurals, either one can be drawn on to make the compound and since what gets eaten, in our example, is in the real world a plural, we get mice-eater instead of mouse-eater (the latter would be used only when the speaker clearly had in mind that the eater was victimizing one particular mouse.
One case that occurred to me that seemed on the line was “low-life”, a person of dubious character. Do we say, “You shouldn’t hang around with low-lifes or low-lives? If we have some old Life magazines lying around, we will definitely ask someone to put away the Lifes, not the Lives. But what about low-lifes? (betraying my own preference). They are, in a way, lives, but they are people, so do they qualify for the regular s plural?