。
She's been a little irritable lately.
Literal
She [topic-は] recently a-little irritably-doing is.
いらいらする pairs the 擬態語 (state mimetic) いらいら ('prickly, edgy, irritated') with する ('do/be'), forming a verb that names the inner state. The reduplicated いら captures the prickling repeat-feeling of irritation — Japanese mimetics encode emotion through sound texture far more than English does. 最近 ('recently') paired with the progressive ~ている locates an ongoing state in the recent stretch leading to now. 少し ('a little') hedges the intensity, framing the irritability as low-grade rather than full-blown.