Is 'ali' an irregular plural?
I am looking at the text of the Verdi aria "D'amor sull'ali rosee", and I am curious about the grammar.

1. The base word is 'roseo' and 'rosee' is the feminine plural, right?
2. The base word 'ala' is feminine singular, right?
3. I should expect the feminine plural of -a, to be -e, right?
4. But instead of 'ale' I am seeing 'ali'. So is this an irregular or unusual plural? If so, can you give me other examples of -a nouns with -i plurals?

(You are welcome to answer in Italian as well as English.)

Question

Is 'ali' an irregular plural?
I am looking at the text of the Verdi aria "D'amor sull'ali rosee", and I am curious about the grammar.

1. The base word  is 'roseo' and 'rosee' is the feminine plural, right?  
2. The base word  'ala' is feminine singular, right?  
3. I should expect the feminine plural of -a, to be -e, right?  
4. But instead of 'ale' I am seeing 'ali'. So is this an irregular or unusual plural? If so, can you give me other examples of -a nouns with -i plurals?

(You are welcome to answer in Italian as well as English.)

Answer

1. Yes
2. Yes
3. Yes, that's the regular declension.
4. It's not totally correct. In ancient Italian we had the form "ale" (singular), which nowadays turned into "ala". For "ale" we should expect the plural "ali". So, we can say that the singular form changed into "ala", while the plural form kept its ancient form "ali". The same happens with "arma/armi", which also used to be "arme/armi".

Answer

1. yes
2. yes
3. yes
4. Una noce, alcune noci
Una voce, alcune voci
Una luce, alcune luci
This is a rule. A feminine word which singular ends with "e", ends with "i" on plural.
About words ending with "a" and having "i" on plural, as ala-ali, at the moment I can think only one word: arma-armi.

Answer

Hello Bill. You answered yourself correctly!
1. Yes
2. Yes
3. Usually yes
4. Yes, it's an irregular plural.

Similar to "ala/ali" I can only think about "arma/armi" (weapon).

But there is a number of words related to body parts which are irregular in different ways.

- La mano, le mani (hand). Feminin articles are used, but the final letters follow the masculine nouns' rules. 
- Il braccio, le braccia (arm). The singular is masculine, the plural is feminine but a bit mixed up.
- Il labbro, le labbra (lip). Same here.
- Il dito, le dita (finger). Same.
- Il ginocchio, le ginocchia (knee). Same
- Il sopracciglio, le sopracciglia (eyebrow). Same.
- Il corno, le corna (horn). Same. Not a human body part, but still a body part.
- L'orecchio, le orecchie (ear). The singular is masculine, the plural is feminine.