Which term would best describe someone who is 'easily angered'?

The term "irascible" is an ideal fit for someone who is easily angered. It specifically denotes a tendency to become easily provoked or irritated, capturing the essence of quick temper and irritability. This word is often used in contexts where a person's emotional response to situations is sharply heightened, indicating a volatility often associated with anger.

The other options present contrasting qualities. "Calm" and "peaceful" suggest a state of tranquility and lack of agitation, directly opposing the concept of being easily angered. "Friendly" implies warmth and sociability, which doesn't align with the notion of irritability or a short temper. Thus, "irascible" accurately conveys the meaning sought in the question.