An IBKR IRA Margin account is allowed to trade some types of American-style option credit spreads. This opens up the possibility of being assigned early on a short call held in an IRA Margin account. IBKR is unable to stop the delivery of short stock into an IRA account from an assigned short call processed by the OCC. The earliest you would see a short call assignment to the account is after the IBKR system overnight refresh which is completed sometime between midnight and 2 AM EST. After the option assignments are processed a notification is sent to your Message Center and you can see the option assignment transaction in your Trade Confirmation report for that day.
A short stock position held in an IRA account is in violation because it would constitute a loan from IBKR which is never allowed. You can enter a closing Limit order starting at 04:00 EST (be sure to configure the order to trade outside RTH) to close the assigned short stock position. If an account is assigned short stock overnight and is still holding the short position after 09:30 EST, the IBKR Liquidation system will start closing the short stock position at 09:40 EST.
If you had an open in-the-money vertical spread, IBKR will not exercise the long in-the-money option to close stock delivered from an overnight option assignment. However, the account holder can use the TWS Option Exercise Tool to make such a request (accessible from Classic TWS/Mosaic or the IBKR Mobile). The Option Exercise Tool is available during pre-market hours to exercise in-the-money long US stock options that will close an assigned short stock position. Depending on the settings selected for the Option Exercise Tool, the user may also have to select "Make Final" to show the stock from the exercise immediately.
Information regarding how assignments are handled by IBKR can be found on the Interactive Brokers website.