The firm had not completed the transaction yesterday, and it could not be established if its traders bought any shares on O'Brien's behalf.
Today, shares in IN&M were up 2.25% at €3.64 (£2.46) in reaction to the news.
O'Brien paid £38.3m when he bought an initial 3% stake in the newspaper group last year. He spent a further £33.9m increasing that to just above 5% in January and February this year.
The IN&M chief executive is billionaire Sir Anthony O'Reilly, who clashed with O'Brien in 2002 when he led a consortium that foiled O'Brien's attempt to buy Irish telecommunications company Eircom and take it private.
O'Reilly holds 28% of the group, but there is speculation that he may step down this year, after IN&M completes the leveraged buyout of Australian media player, APN, in which it holds a 40% interest.
IN&M owns The Independent and The Independent on Sunday. It operates in Ireland, Britain, South Africa, Australia, New Zealand and India.