Translation:Is leatsa é.
I wanted to write down the same thing, but then remembered that in these cases "yours" is constructed in Irish as a "prepositional pronoun" with a similar structure in all the possessive pronouns, i.e. beginning with a form of the preposition "le" ("with"): in the case of the plural "yours" it is"libhse". Here you will find all the forms, at the bottom of the post: http://www.bitesizeirishgaelic.com/blog/i-me-he-him/.
Modern English does not make any distinction, by and large, between 'yours' your singular and 'yours' plural yours. So, Is libh e --- is correct, it refers to a plural you. As commented before, can't see any justification for the course using emphatic forms liomsa as opposed to liom, leatsa as opposed to leat, so Is libh e --- should be accepted --- beir beannacht
Not really what I was referring to: namely Modern Standard English does not distinguish 'You' singular and you 'Plural'. So, take "it is mine": Is liomsa e -- as the course prefers, but, it is yours, when it comes to translating is ambiguous, you singular or you plural -- the English does not make a distinction, therefore, it is justifiable to use either Is leatsa e or Is libhse e or, leat or libh. Thus, to be told it is wrong to use the plural you 'libh' in the construction is unfair and untrue, for some 'its' will be owned/possed/belong to a plural YOU. Sin e --- beir beannacht agus slan