Weak and strong plurals
How can weak and strong plurals be identified? I'm asking because I think you need to know them in order to know how to form the genitive plural. Also, how do you form the genitive plural once you've learned how to identify strong and weak plurals?
Weak plurals are where you slenderize, or add an a to form the plural. Strong ones are anything else. See here.
With weak plurals, the genitive singular is the same form as the nominative plural, and the genitive plural is the same as the nominative singular.
Strong plurals: the genitive singular is the same as the nominative singular and the genitive plural is the same as the nominative plural.
Those aren't always right péitseog to péitseoige for example (singular)
Something to note is that plurals can vary by dialect.
Take, for example, fuinneog. In standard Irish, the plural is fuinneoga, making it a weak plural. In Connacht Irish, however, it's fuinneogaí, making it a strong one.
Or, leabhar, which is pluralized to leabhra instead (and, from what I can tell, leabhar is the genitive singular), etc.
I use a slightly different division — strong plurals are where the nominative plural is the same as the genitive plural and (ignoring lenition) the vocative plural, and weak plurals are anything else. This categorization accurately identifies e.g. súile (< súil ) and glúine (< glúin) as being weak plurals.