A quick google search for  "whats included in IIRMA MAGI?"   shows this as the first paragraph at the top of the hits

IRMAA is based off of what is called your Modified Adjusted Gross Income, or MAGI. This can be confusing to calculate or find because it is not a specific line item on your tax return. To calculate MAGI for Medicare IRMAA, take you AGI and add tax-exempt interest.