Irritability is one of the many symptoms of ADHD.
Trust me on this.
From a big article on ADHD:
Wender (1998) provided a list of adult behaviours linked to childhood ADHD (see also Weiss & Murray, 2003; Asherson, 2005). Motor hyperactivity may be replaced by a subjective sense of restlessness, difficulty in relaxing and settling down and dysphoria when inactive. Attention deficits may well persist in a lack of concentration on detail, the need to re-read materials several times, forgetting activities and appointments, losing things and losing the thread of conversations. Thoughts are unfocused and ‘on the go’ all the time. Mood changes are often rapid shifts into depression or excitability, irritability and temper outbursts that interfere with personal relationships. Disorganisation is prominent, tasks are not completed, problem-solving is lacking in strategy and time management is particularly poor. Impulsivity continues and leads to problems in teamwork, abrupt initiation and termination of relationships, and a tendency to make rapid and facile decisions without full analysis of the situation.
Although most people experience such symptoms at times, individuals with ADHD experience these to a severe degree most of the time.
I impulsively decided to put that sentence in bold.