Грамматика русского языка. Часть 6. Падежи. Глава 5. Родительный падеж, множественное число, “неправильные” существительные. Russian Grammar. Part 6. Cases. Unit 5. The Genitive Plural Irregular.

There are a number of very common nouns which have irregular forms in the genitive plural.
The important thing is to know which nouns have irregular nominative plurals, because their genitive plural forms will be based on this.

This is what happens to masculine nouns in the genitive plural: if the nominative plural ends in stessed а, then the genitive plural ending is ов (so not strikingly irregular, just be aware of the stressed ending).
If the nominative plural ends in:

• ья and the word is stressed on the stem, the genitive plural ending is ьев;
• stressed ья, then the genitive plural ending is ей; 
• не, then the genitive plural ending is ... nothing!

Nominative singular	Translation	Nominative plural	Genitive plural
город	town	города	городов
стул	chair	стулья	стульев
друг	friend	друзья	друзей
англичанин	Englishman	англичане	англичан

There are very few irregular genitive plural endings for feminine nouns. The most common are the words for mother and daughter:
мать – матерей, дочь – дочерей
NB Some feminine nouns with a “cluster of consonants” before their ending form their genitive plural by inserting the letter ё:

звезда (star) - звёзд, cестра (sister) - cecтёp, cepьга (ear-ring) – серёг

Although a number of common neuter nouns have irregular nominative plural forms, only a few have irregular genitive plurals, e.g.:

время (time) - времён, уxo (ear) - yшей, имя (name) - имён

And finally, the genitive plural for “children” and “people” comes from their strikingly different nominative plural form:

Nominative singular	Nominative plural	Genitive plural
ребёнок	дети	детей
человек	люди	людей

NB After a numeral the genitive plural of человек is человек (5 человек, 5 people).