。
She wasn't irritated in the slightest.
Literal
She [topic-は] at-all irritated was-not.
いらいら is a state mimetic (擬態語) for the prickling feeling of irritation or impatience. Combined with する ('to do/feel'), it becomes いらいらする ('to feel irritated'). The 少しも + ~ていなかった double-negation emphasizes complete calm.