Sorry if it’s a stupid question but this is confusing me. If it’s literally ‘I have knowledge’ why does the ‘a’ come into play? ‘I have a car’ is just ‘Tá carr agam’, no ‘a’ involved.

A refers to what you know and anticipates that clause:

a fhios agam go ndéarna tú é.

If such a clause is missing, a represents the missing clause.

a fhios agam. = I know it; literally: I have its knowledge.

In English, “I know” (without “it”) can stand alone but is elliptical (“I know ….”). In Irish, “Tá a fhios agam.” is a full sentence.

EDIT: Random other question that popped into my head: Why is it ‘ní féidir’ instead of ‘ni fhéidir’?

Because ní is a copula form here in present tense. And all present tense copula forms cause neither lenition nor eclipsis:
Is féidir, ní féidir, an féidir, nach féidir, gur féidir, más féidir …

Or in other words: ní féidir is rather ní [is] féidir. The omitted (is) in parenthesis prevents the verbal particle ní from causing lenition.

(BTW: The forum software sucks. Why must I make two empty lines to get one empty line? )