A hallmark of Early Modern English is that it exhibits a lot of variance between the use of final -y and -ie. In the 16th century -ie is even found in Old English words, eg stonie. And Mulcaster in his Elementarie of 1582 effectively recommends the dominant use of -ie when he relegates final -y only to "sharp and loud" final vowels (eg, deny but prettie). Why did -ie become so widely used in the 16th century? Are there phonological reasons to select -ie over -y? Is it due to contemporary French practices? I assume it's a complex mix of causes - if so, is there a study into the "rise and fall" of -ie?
Phonetically they are identical nowadays, but they probably were pronounced differently in the past. I know that, at one time, in French the pronunciation of -ie was different than -i, and still is in poetry (the -ie indicating the feminine form of a word). The ending -y is rarely ever found in French.
According to The Guardian, the -y and -ie suffixes have different origins in English:
y or ie? As a general rule: -y is an English suffix, whose function is to create an adjective (usually from a noun, eg creamy); -ie was originally a Scottish suffix, whose function is to add the meaning of "diminutive" (usually from a noun, eg beastie).
So in most cases, where there is dispute over whether a noun takes a -y or an -ie ending, the correct answer is -ie: she's a girly girl, but she's no helpless girlie. Think also scrunchie, beanie, nightie, meanie ... There are exceptions (a hippy, an indie band), but where specific examples are not given, use -ie for nouns and -y for adjectives.
Questia has a link to an article excerpt by Kenneth Shields titled "On the Origin of the English Diminutive Suffix -Y, -Ie", but the complete article is only available to subscribers.