Any emergence of mood changes (irritability or dysphoria) or agitation is very relevant and should be reported to the doctor! There a many possible causes that could contribute to this change.
Titration of the right dose
The medication needs to be monitored and adapted to the personal needs. So the child might need a higher dose of the medication or might be very sensitive to psychostimulants. This should be evaluated!
Normal methylphenidate pills work for about 3 to 4 hours. After that time a "rebound" of the old symptoms can occur. The patients feel very uncomfortable with this. Adaptation of the right dose and interval is necessary or you might try long-acting psychostimulants instead.
Change stimulant preparation
Some children do not respond to methylphenidate but do fine with amphetamines.
Evaluate comorbid disorders like depression or anxiety disorders!!!
Many adhd children have severe problems at night. If they cannot sleep they might feel irritable and dysphoric.