The correct way to localize allowance amounts in Workday without impacting other populations is through the use of plan profiles. Plan profiles allow administrators to define country-specific or location-specific values using eligibility rules, ensuring that employees automatically receive the correct amount during hire and other staffing events.

In this scenario, adding a new plan profile for Ireland with a value of €90 EUR ensures that employees hired in Ireland automatically receive the correct allowance. This approach preserves the existing default value of $100 USD for employees in the US and avoids unintended downstream impacts.

Updating the default value would affect all employees who do not meet profile eligibility, including US employees. Allowance Plan Adjustments are used to update existing assignments, not to control defaulting logic during hire. Manual compensation changes are inefficient, error-prone, and contradict Workday best practices for scalable configuration.

Therefore, adding a new allowance plan profile for Ireland is the correct, controlled, and Workday-recommended solution, making option C the correct answer.