Tension Headache: This is often the most common cause of headaches, including those localized to the occipital region. Tension headaches are typically characterized by a band-like pressure around the head, but can also present with pain localized to the back of the head. The irritability associated with the headache could be due to the discomfort and stress exacerbating the headache.

Cervicogenic Headache: These headaches are referred from the cervical spine and can cause pain in the occipital region. The irritability could be related to the underlying cervical issue causing both pain and discomfort.
Occipital Neuralgia: This condition involves irritation of the occipital nerves, leading to pain in the back of the head. The pain can be severe and is often described as sharp or stabbing, which could contribute to irritability.
Migraine: Although migraines typically present with unilateral pain, they can occasionally cause occipital headache. The associated symptoms like photophobia, phonophobia, and nausea could contribute to the patient's irritability.

Subarachnoid Hemorrhage: Although less common, a sudden, severe headache (often described as "the worst headache of my life") could be indicative of a subarachnoid hemorrhage. The sudden onset and severity could cause significant distress and irritability.
Meningitis: Inflammation of the meninges can cause severe headache, often accompanied by fever, neck stiffness, and photophobia. The severity of the symptoms could lead to irritability.
Temporal Arteritis: This condition, more common in older adults, involves inflammation of the blood vessels and can cause severe headache, often localized to one side of the head but can be occipital. The pain and associated symptoms like jaw claudication could contribute to irritability.

Chiari Malformation: A structural defect in the cerebellum that controls balance can cause headaches, often occipital, due to the compression of the cerebellum into the spinal canal. The chronic nature of the condition could lead to irritability.
Eagle Syndrome: This condition involves an elongated styloid process or calcified stylohyoid ligament, which can cause sharp, stabbing pain in the side of the face and the back of the head. The unusual and severe nature of the pain could contribute to irritability.
Giant Cell Arteritis: Similar to temporal arteritis but can affect other large vessels, leading to a variety of symptoms including headache. The systemic nature of the disease and its potential to affect vision could cause significant distress and irritability.