Should it not be 'an cháis' since it's a feminine noun? Or is there some wibbly-wobbly grammar-wammar reason for this?
It should be. Please report it as an error so they can fix it.
It’s fixed as of 2016-09-01.
When exactly is "bhfuil" necessary? I don't get it.
When using it as a question in the present (non-habitual) tense.
i thought if the word was feminine it was NOT lenited???
It is exactly the opposite: when a noun is feminine it will be lenited when it is definite (ie when the definite article 'an' appeares before it)
Before the article an, both feminine nouns in thenominative case, and masculine nouns in the genitive case. Tricky.