Being irritable is known by most people to be a result of tiredness. Many parents know it to be a result of tiredness, hunger, pain, discomfort, illness, not getting his or her own way, etc.
But when a person is also on the Autistic Spectrum, those niggling causes often produce much more intense irritability.
A person with ADHD or Aspergers Syndrome for example, is likely to have hypersensory perception (HSP) that makes them have intense dislikes and be extremely intolerant of certain loud noises (or specific noises), being touched, certain smells, etc. When they experience one of those things that trigger their irritability, they might behave like a someone else would behave if they were attacked by a swarm of wasps; – hysterically.
That would be an extreme example, but most people on the autistic spectrum who also have anxiety, anger or depression issues, will almost certainly have triggers that make them more irritable than most other people not on the spectrum.
This might make them more inclined to raise their voice, when they feel they are not being understood. They could become tearful or angry in a frustrating situation, creating an outburst or panic attack.
At the very least, they might appear impatient. Or they could simply appear rude or abrupt, and might not even be aware that’s how they appear.