The pair's investment vehicle, Elpida, notified the stock exchange yesterday that it has increased its holding in the business -- one of Britain's biggest pub chains -- by acquiring just over 7 million shares. That's likely to have cost the businessmen around £20m (€25m). They previously owned just over 20pc of the company.
The acquisition of additional stocks has brought Elpida's holding to almost 92.1 million shares in M&B, whose 1,600 pubs trade under names in Britain such as O'Neill's, All Bar One and Harvester. That stake is valued at about £276m (€344m).
The Irish investors increased their holding in M&B after the company said last week that like-for-like sales growth rose 3pc in the nine weeks to September 15.
However, it warned that its full-year margins will still probably be behind those it generated in 2011, as a result of inflationary cost increases and higher investment in its portfolio.
Mr Magnier and Mr McManus first invested in M&B in 2007 and have accumulated shares since then. The last time they bought shares in the company was in 2011.
Elpida is the second biggest shareholder in M&B after Piedmont, the investment vehicle owned by Bahamas-based billionaire Joe Lewis. He owns 26.3pc of M&B. Last year, Mr Lewis unsuccessfully tried to engineer a takeover of M&B.
He made a £2.30 a share offer for the pub group in September but was lambasted for trying to take control at what many investors thought was too cheap a price. The shares were trading at about £3 yesterday. Mr Lewis withdrew his offer in October last year. He had also been hoping to persuade Elpida to join him in his takeover effort.
Under takeover rules Mr Lewis had to wait at least six months before he could table another offer for the group.
M&B has endured a chequered recent history.
In early 2009 it revealed a £500m loss due to a failed derivatives bet. Later that year, a feud erupted between some board members and investors, including Mr Magnier and Mr McManus, while Mr Lewis accused M&B of failing its shareholders.