The 72-year-old star also made reference to the campaign by grime stars including JME - which aims to encourage young people to vote and get Corbyn into number 10 - in his public service announcement, writing: "show us how it's done!#grime4corbyn".
One of the world's best-loved film stars has endorsed the Labour leader in the United Kingdom general election.
"In particular Labour will ensure there is no return to a hard border between Northern Ireland and the Republic of Ireland and that there is no change in the status or sovereignty of Gibraltar", the partY said in the manifesto.
Jeremy Corbyn and his team will take comfort in the poll, which suggests that the Labour vote is solidifying as the campaign advances.
"I believe these next three weeks will throw up something that is quite extraordinary".
In a sign of the pressures within the Labour camp as it continues to trail in the polls, he told the Politico website: "I don't see Labour winning. There is everything to play for now and we'll be fighting for every single vote".
"The scale of the task is enormous".
His comments were dismissed by Labour's elections co-ordinator Andrew Gwynne, who said: "This is absolute rubbish from the Tories and yet another wholly cynical ploy to try and avoid scrutiny of their own spending plans". To be frank, don't believe them.
"I think he [Mr Corbyn] would make a great PM". We'd like to hear from people who are planning to vote for one of the main political parties - Labour, the Conservatives or the Liberal Democrats - for the very first time.
Mr McDonnell also told the BBC it was not true that Mr Corbyn was unpopular with the electorate, given how popular his policies were.