EXHIBIT 23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We consent to the incorporation by reference in this Registration Statement on Form S-4 of our reports dated February 28, 2005, relating to the consolidated financial statements of Marshall & Ilsley Corporation, and management’s report on the effectiveness of internal control over financial reporting appearing in the Annual Report on Form 10-K of Marshall & Ilsley Corporation for the year ended December 31, 2004 and to the reference to us under the heading “Experts” in the Prospectus, which is part of this Registration Statement.

/s/    DELOITTE & TOUCHE LLP

Milwaukee, Wisconsin

November 21, 2005