IRS interest accrues quarterly. It can add as much or more than the penalties applied to an unpaid tax account. Only future interest can be reduced. Any interest applied to an IRS debt cannot be waived. (Be careful of any firm that claims otherwise!) However, if we find a way to reduce the principal owed to the IRS (amend a filing, for example), the interest accrued from the principal that is negated will be negated with it.