The Irish racecourse magnates are now the club's two biggest shareholders. Through their investment vehicle The Cubic Expression company, the pair now own 11.4 percent of the Premiership champions.
They previously held a 10.37 percent shareholding in the club.
A United statement read: "Manchester United has today been informed that The Cubic Expression Company has increased its holding in the company and now holds 29,612,396 ordinary shares representing approximately 11.4 percent of the issued share capital of the company."
The club's shares fell nearly 4 percent last week after a report that the two Irish tycoons, close associates of team manager Alex Ferguson, might sell their stake.
Shares have since risen 10 percent on the back of reports of interest from other investors and a possible takeover.
Contrary to reports, Irish duo JP McManus and John Magnier have increased their joint stake holding in English soccer club Manchester United.
