I don’t believe there is a specific transformer that will calculate IRI.
If you know the formula however, you can use either the AttributeManager or AttributeCreator transformer to create a new attribute and enter the formula to set the value. This can be done by selecting the Arithmetic Editor from the Value column.
In terms of writing the formula in the AttributeManager this link might help you out with setting up the Arithmetic Editor in the value column (under number 10).