"This story is not true."
Translation:Ní fíor an scéal seo.
Also, I believe that it would need to be an exclamation with a subjective judgement to use a predicative adjective with the copula — Ní fíor é an scéal seo!. Otherwise, Níl an scéal seo fíor. would be used for a statement, whether subjective or objective, and Níl an scéal seo fíor! would be used for an objective exclamation.
I think I've asked this somewhere else but I can't find it back. I see on teanglann that fíor is both a noun and an adjective.
Can I assume that when using the copula, it is a noun (i.e. "This story is not (a) truth") while when used with bí, it is an adjective (i.e. "This story is not true")?
Thanks for clarifying the missing é with the copula too.
You will encounter Is fíor é and tá sé fíor for "it's true", as well as is fíor sin, is fíor é sin and tá sé sin fíor for "that's true".
Because of it's dual nature as both an adjective and a noun, it's not a good model to use for constructing other sentences (and it's worth noting that, even in English, "that's true!" and "that's the truth!" are largely equivalent).
For your other examples, the form that I'm familiar with would allow you to say Is gránna an madra é ("it's an ugly dog"), or is tuirsiúil an caint í ("it's tiresome talk").
I would see Is gránna é an madra seo! as an example of elision (Is madra gránna é an madra seo!)
Using fíor as an adjective, you don't use the copula, and then you get níl an scéal seo fíor, using the negative form of tá an scéal seo fíor.
Using fíor as a noun, you use the copula - ní fíor an scéal sin.
I think it was a mistake for Duolingo to accept it as a typo - if you meant to use the copula, then you can't have an scéal next to it. If you meant to use the verb bí instead of the copula, then it has to be níl, not ní.
I thought it should be "Níl an scéal seo fíor." but it was a question where you are given the options for words and "Níl" was not one of the options but "Ní" was, so I chose the words "Ní an scéal seo fíor." and was told it was incorrect and it should have been "Níl an scéal seo fíor." BUT "Níl" wasn't an option to choose!!
This has already been explained in considerable detail in the earlier comments. "This story is not true" can be translated with the copula statement Ní fíor an scéal seo or the non-copula Níl an scéal seo fíor. If "Níl" was not one of the options but "Ní" was, then you have to give the answer that uses the copula.