Leo Varadkar said the bloc would have the “stronger team” because of its sheer size and influence globally.
He also warned that the EU would not accept a “piecemeal” trade deal in which the UK had rights without obligations.
“The European Union is a union of 27 member states. The UK is only one country. And we have a population and a market of 450 million people,” the taoiseach told the BBC.
“The UK, it’s about 60 million. So if these were two teams up against each other playing football, who do you think has the stronger team?”
Mr Varadkar is meeting with the EU’s chief negotiator Michel Barnier in Dublin in Monday. It comes amid a general election campaign in Ireland.
While the taoiseach’s approach to the Brexit negotiations was widely supported in Ireland, his party has seen its popularity fall since the issue came out of the spotlight.
Britain’s government says it wants a strong free trade agreement that would guarantee it zero tariffs and quotas with the EU market.
But Boris Johnson has set a strict time limit of the end of the year for the signing of a deal, and also laid down a number of red lines that could prove difficult.
Mr Johnson’s government has been reticent to sign up to a full slate of “level playing field” provisions that would strongly tie the UK to EU rules in the future.
But Brussels and member states say that a level playing field on areas like the environment, labour rights, and manufacturing standards is necessary to guarantee that UK producers can’t undercut European businesses when they’re given access to markets.
The UK is worried that being too closely tied to EU rules will make it harder to negotiate a deal with the US, which demands its own regulations are followed. Some cabinet ministers also want to deregulate for its own sake.
The issue is likely to be a dividing line in the upcoming talks, which will kick off over the next month.
“When I hear people talk about piecemeal, it sounds a bit like cake and eat,” Mr Varadkar said. “That isn’t something that will fly in Europe.”