It's worth trying your best with the irregular plurals, so hopefully the lists offered by other members will help. There are two basic reasons that a noun may have an irregular plural:
1) The noun follows old Anglo-Germanic rules. There exceptions are common daily things (especially to a working farmer): men, mice, geese, fish, sheep, lice, oxen, brethren...
2) The noun is a borrowed word, and also borrows the plural form from the other language. "Cacti" and "fungi" come to mind. Trying to make an English plural by adding "s" may sound awkward.
And then there are the uncountable nouns...