Fòram na Gàidhlig

I'm still trying to understand all the structures associated with the copula and I'm confused about when you can use an adjective following 'is'.
example: Is bòidheach am feasgar.
I thought if it was anything other than noun, pronoun, noun phrase you would use 's ann ie: 'S ann bòidheach a th' am feasgar.

Think of these Is / Nach + adjective phrases (for now at least) as idioms, rather than as the standard structure.

There are some useful more-or-less fixed phrases that you come across more commonly, and can safely use, e.g.

Nach bochd sin!
'S math a rinn thu! Nach math a rinn iad!

and you'll see others, especially with bòidheach, muladach or other emotionally charged adjectives in songs and poems, which can be seen as historical use or literary device.
I would avoid being creative with such constructions until you really have a good feel for style and register in Gaelic.

Even the 's ann + adjective as emphatic "fronting" (though standard as a structure) shouldn't be overused. It can sound a bit formal or poetic.
My observation is that Gaelic likes nouns (the way English likes verbs) and it would be more common to say

'S e feasgar dìreach àlainn a bha againn a-raoir
than
'S ann àlainn a bha am feasgar.

My feeling is that 'S ann tends to get used most for contrast, and is especially common with prepositional phrases, rather than adjectives.

'S ann ruadh a tha falt Màiri, chan ann bàn.
'S ann aig Màiri a bha an leabhar, chan ann aig Seumas.
'S ann anns an Fhraing a thachair e.

Good advice

In English, you can just raise your voice to put emphasis on a word, and the stress system is pretty intricate. You can't do that in Gaelic at all, so you have to use grammatical devices like extra words instead. 'S ann is one of those devices.