If you have been named in a financial power of attorney document, you serve as the agent for the person who named you, the principal. Your powers are limited by the terms of the legal document. If the principal created a general power of attorney without limiting language, you have access to all of her funds, including IRA accounts.
Financial Power of Attorney
A financial power of attorney is a legal document, prepared by a competent adult, giving another adult the authority to act for her in financial matters. To determine the scope of your authority under the power of attorney, read the document. The person making the document can limit the agent's power in any way she decides best fits her needs.
Changing IRA Ownership
If the power of attorney naming you agent is a general one, you have authority to take any action that the principal could have taken herself. This would allow access to retirement accounts such as IRAs. However, changing the legal ownership of an IRA might well be beyond the scope of federal and state retirement account laws. If the principal did not have the right to do it when competent, neither do you.
An agent appointed under a power of attorney owes the principal a very high duty of good faith and fair dealing, which is called a fiduciary duty. Remember that you must act at all times in the principal's best interest and strictly avoid self-dealing.