It's not that most masculine nouns have irregular plurals (so that you have to memorize a different plural for each noun, though after a while they start to fall into a few common patterns). It's not that there's no regular form for the infinitive (so that you have to memorize a different infinitive for each verb). No, this is crazier.
In Arabic, as in many languages, every noun has a gender. Verbs, adjectives, pronouns, etc., have to agree with their subjects in gender and number (though Charley can tell us about an important exception). Fine. I'm used to that. That's not the crazy part.
When nouns become plural, one would expect them to keep the same gender (or perhaps lose their gender entirely, as we see in the limited examples of grammatical gender in English: he/she/they). And in Arabic, that is indeed what happens for animate nouns (nouns that describe people). However, for inanimate nouns, regardless of whether the singular is masculine or feminine, the plural is treated (for the purpose of verb/adjective/pronoun/etc. agreement) as feminine singular.
boy = masculine singular
boys = masculine plural
girl = feminine singular
girls = feminine plural
bus = masculine singular
buses = feminine singular
car = feminine singular
cars = feminine singular
(And this means that all of the plural pronouns, verb forms, etc. are only used when talking about people.)
However, numbers are gendered based on the gender of the singular noun. Thus, "three boys" and "three buses" have the masculine form of "three", and "three girls" and "three cars" have the feminine form of "three".
Also, collective nouns are generally masculine singular, but when you're talking about a single item, it becomes feminine.
apples (collectively, like English "rice") = masculine singular
apple = feminine singular
(three) apples = feminine singular (since it's an inanimate plural)