It's irritating to wait a long time for a friend to show up.
Literal
Friend [subject-が] appear [nominalizer-の] [object-を] long time wait [conditional-と] get-irritated.
いらいら is a 擬態語 (state mimetic) for the feeling of irritation or impatience — a buzzing, prickling frustration. The ~と conditional creates a general-truth statement: 'whenever you wait a long time, irritation follows.' のを nominalizes the clause 友達が現われる into the object of 待つ.