Final answer:
In the active phase of labor, a nurse's first action is to assess vital signs and provide comfort measures to the mother.
Step-by-step explanation:
A client who is 8cm dilated and is experiencing increased irritability and discomfort is in the active phase of labor. The first action a nurse should take is to assess both the mother's and the baby's vital signs to ensure that there are no signs of distress or complications. It is important to monitor the progress of labor closely and provide comfort measures to the mother, such as pain relief options if available and necessary, position changes for comfort, and emotional support.