A close ally of the Irish prime minister is poised to get a key role in post-Brexit negotiations in Brussels as the new commissioner for trade.

Phil Hogan, the agriculture commissioner, is expected to be formally appointed to the role of EU trade commissioner on Tuesday by the incoming commission president, Ursula von der Leyen, and will take up his role on 1 November.

In his new job Hogan will represent the EU at the World Trade Organization and oversee talks in a move that could sound alarm bells for Brexiters hoping for a swift trade deal.

But his appointment could also be a blessing for Leo Varadkar, the taoiseach, who told Boris Johnson that Ireland would be the UK’s closest friend and ally in trade negotiations.

Hogan, 59, is from the same party as Varadkar, Fine Gael, and was appointed after being nominated by the taoiseach’s predecessor, Enda Kenny.

Announcing Hogan’s appointment, Von der Leyen said: “He will be a very fair, but determined negotiator.”

Von der Leyen also reiterated that the manner of Britain’s EU departure would shape its future relationship with the bloc. “Brexit, should it happen, is not the end of something, but it is the beginning of our future relationship.”

Von der Leyen, a former German defence minister, has previously told the Guardian that she was “open to listening” to a request for extension “if there are good reasons”.

Known in Ireland as “Big Phil”, Hogan is a fixer, bruiser and wily operator. The Irish Times recently described him as “a sometimes abrasive political figure at home” but “also a deft political strategist” who had recently come under attack from farmers in Ireland over a proposed beef deal with countries in the South American Mercosur bloc.

He has been highly critical of the British approach to Brexit and recently he launched a scathing attack on Johnson, describing him as an unelected prime minister who was “gambling” with Ireland’s peace process.

He also warned that a hard Brexit would create a “foul atmosphere” with the EU and would have “serious consequences” for the UK’s chances of a favourable trade deal with the bloc.

However, others in Ireland see his appointment to the trade portfolio as a positive step for Johnson because the taoiseach and his deputy have repeatedly promised over the past three years that they would be the UK’s strongest ally in trade talks, because no country wants a better outcome than its westerly neighbour.

On Monday, Varadkar warned Johnson that trade talks with the EU would be a “herculean” challenge but said Ireland would act as his friend in Brussels. “We want to be your friend and ally, your Athena,” he said in reference to the Greek goddess who prevented Hercules from murdering his foster parent.

Von der Leyen’s commission will be the first not to include a British member since 1973. The remaining 27 EU member states have nominated a politician to send to Brussels. Only the UK did not propose a candidate, in line with Johnson’s determination to leave on 31 October “do or die”.

But Von der Leyen said the UK would have to appoint a commissioner if it was still in the EU after 1 November.

European commissioners swear an oath not to take instructions from any government and act in the general interest of the bloc. The UK’s current commissioner, the career diplomat Sir Julian King, was given a job to promote the EU’s “security union”, after Lord Hill quit his post on financial services days after the 2016 referendum result.