The Minister for Arts, Sport and Tourism has said Fianna Fáil faces major difficulties in its efforts to retain six seats in the European election.
Mr O'Donoghue, who is Fianna Fáil director of elections in the European poll, said the reduction to 13 from 15 in the number of Irish seats in the parliament left the party with a "daunting" task.
"Nobody really expects us to hold six out of 13," he said, indicating that such views were held within Fianna Fáil and outside it.
Mr O'Donoghue identified particular challenges in the South and East constituencies, the former four-seaters which are now three-seaters due to the reduction in seats.
He also identified difficulties in the North-West constituency, where recent private polls suggested that the second Fianna Fáil candidate, Mr Seán Ó Neachtain, is unelectable.
"Obviously we're trying to hold six seats out of 13. We held six seats out of 15. It's a daunting task, but we've put an awful lot of energy and effort into it," said Mr O'Donoghue.
"We've a very professional campaign now under way, so we're going to give it a reasonable shot. If we don't make it, well, we'll have given it our best shot."
Mr O'Donoghue said there would be a "dog fight" for the third seat in the South constituency, formerly Munster, where Mr Gerry Collins and Mr Brian Crowley are seeking re-election.
"We're confident that we should be able to hold our two seats in Munster, after a difficult struggle," Mr O'Donoghue said.
Of the North-West constituency, he said: "We're in a fight for the third seat there. It's going to be very difficult." Mr O'Donoghue said he had not decided whether the party's two candidates in Dublin should divide the constituency between south and north sides of the city.
While Fianna Fáil is dividing each of the other constituencies between its candidates on a geographical basis, a row has broken out between the two Dublin candidates over the merits of such a strategy.
Mr Eoin Ryan TD wants to campaign on both sides of the city, but the Lord Mayor of Dublin, Mr Royston Brady, is opposed to such a plan. Mr Brady wishes to confine his own campaign to the north city and wants Mr Ryan to campaign only on the south side.
Mr O'Donoghue said: "With a fair wind to our backs it is possible on the day that we would actually pull off two seats in Dublin."
He said the party would support Mr Ó Neachtain "to the hilt", even though the party leadership refused to back him before a selection convention and supported his rival, Mr Frank Fahy. The issue was now closed, Mr O'Donoghue said.
Mr Ó Neachtain is running with Dr Jim McDaid.