US President Donald Trump lashed out Sunday at Ukraine for targeting Russian oil refineries, vowed to end tariffs on Irish whiskey and played down the threat from Artificial Intelligence on a two-day trip to Ireland which again saw him mix diplomacy and personal business interests.

Trump ended his stay by presenting Irish Open golf trophy to tournament winner Shane Lowry at his Doonbeg golf resort on Ireland's west coast.

The ever-unpredictable American leader said that Ukraine's President Volodymyr Zelensky had to do "one thing" and "stop knocking out diesel fuel" refineries in Russia due to growing shortages.

"Let him go after targets, but not diesel fuel, because he's causing a shortage of diesel," Trump said, insisting the Russia-Ukraine war -- not the US-Israel conflict with Iran -- was to blame.

Oil prices have soared as attacks in the Middle East have intensified, raising American fuel costs and inflation and speculation the Federal Reserve will hike interest rates just weeks before crunch US midterm elections.

Trump also said he would lift tariffs on Irish whiskey following appeals by Ireland's Prime Minister Micheal Martin and golfer Lowry.

Whiskey exports to the key US market have suffered since Trump imposed tariffs of up to 15 percent on European Union products.

He also waded into the growing controversy over the safety of AI, saying that "negative" forces opposed its development.

On Saturday, he caught his Irish hosts and neighbouring Britain off-guard when he announced he would "love to see" a united Ireland.

'Not welcome here'
Breaking with Washington's traditional neutrality on the historically charged topic, Trump argued that reunification of the Republic of Ireland and Northern Ireland -- after more than a century apart -- was "going to happen eventually".

"It may as well happen now," he said as he met Ireland's Prime Minister Micheal Martin in Dublin, prompting pushback from British loyalists in Northern Ireland and some leading UK politicians.

But he reaffirmed his remarks Sunday.

"It would seem to me that one of the naturals of all time is to put them together," he added, insisting "many people agree".

Trump's arrival in Doonbeg on Saturday sparked road closures and police checkpoints. Around 30 people protested his visit in Doonbeg village Sunday.

"Donald Trump is not welcome in Ireland," William Hederman, 53, told AFP, accusing him of helping Israel commit genocide in Gaza.

Trump's attendance at the golf course he has owned since 2014 tournament added extra complexity to a security effort that is reportedly costing Ireland 20 million euros ($23 million).

Spectators cheered when he walked onto the first tee.

'Conflict of interest'
The former real estate magnate, who promotes his enterprises at every opportunity, was joined in the remote corner of Ireland by his sons, Eric and Don Jr.

They have managed the family business since their 80-year-old father returned to the White House last year.

The course -- set among dunes on the rugged County Clare coastline -- has twice been targeted by pro-Palestinian activists in the past 18 months.

Police said three women -- aged in their 20s, 30s and 40s -- had appeared in a western Irish court late Saturday after being arrested for separate trespass and public order offences there.

Around 4,000 Irish and US security personnel were involved in the weekend security operation, with anti-drone technology among the measures deployed.

The president plays golf most weekends at one of his family's US properties. He is scheduled to host December's G20 summit at his Miami golfing resort.

Opposition Democrats have long complained that blurring the lines between his presidency and personal business interests is corrupt.

The White House has aggressively pushed back against claims of conflicts of interest and that Trump has been using his presidency to enrich himself, as his personal wealth has rocketed during his second term.

"Since the Civil War, we haven't had a president with a significant financial conflict of interest from a business enterprise," University of Minnesota Law School professor Richard Painter told AFP.

The former White House ethics lawyer warned Trump's conduct could send the message "that the way to curry a favour with the president is to patronise his properties, do business with his companies and so forth".