Do adjectives always need ’agus’ between them? In Irish you could just say “Níl sé te grianmhar”.
no. You could list them the same way you do in English. For example:
Tha i blàth, grianach, ach gaothach.
You actually don't need the comma like you do in English. Most natural to me would be Tha i blàth grianach.
Would "It's neither warm nor sunny." not feel like a more colloquial translation into English of "Chan eil i blàth agus grianach."?
It read to me almost like a contradiction of something stated earlier, as if the weather forecast had been for a warm and sunny day, and the speaker is looking out the window at the rain and saying, “It is not warm and sunny.”
Maybe, but I'm not sure it would help people translate. People might start trying to rack their brains for a specific word for neither.
Or maybe not. I would never describe that as colloquial, but as formal.
Can blàth mean warm or hot?
I have never heard it used to mean 'hot' and nor has faclair.com. Use teth.