Most of the time, a copula’s predicate is a noun phrase, but there are a couple of situations in which it can be an adjective — an exclamation with a subjective judgement is one of the situations in which fíor can be “true” rather than “truth”.

To my knowledge, any adjective could be used in an exclamatory subjective judgement. (Since caint is feminine, Is tuirsiúil í an chaint seo! would be better.)

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!)