"Finally arrived!" Patrick Lyttle posted triumphantly on Facebook when he touched down in Sydney last June.

It was the trip of a lifetime for the 31-year-old from Belfast, and his older brother Barry was the first to share in the good news.

"Enjoy bro!!" Barry replied, telling their friends he would be heading over to join his brother in January.

In the early hours of Saturday morning, three days after the brothers were reunited, Patrick was fighting for his life in St Vincent's Hospital and Barry was facing serious criminal charges over his alleged assault.

Their devastated father, who joined them on the trip, is at Patrick's bedside. Police said the "pain in his eyes" was indescribable.

Barry, 33, allegedly punched his younger brother in the head on Bayswater Road in Potts Point shortly before 3am on Saturday, causing Patrick to fall backwards and hit his head on the ground.

Police say CCTV footage appears to show the siblings having a "minor" argument before Patrick pushes his older brother, who reacts by throwing a punch. Alcohol has already been ruled out as an "aggravating factor" in the fight.

Patrick, who was treated by NSW ambulance paramedics before being taken to hospital, is on life support after undergoing emergency surgery to release pressure on his brain.

His older brother, whose Facebook page is littered with photos of the pair posing with their arms around each other's shoulders, was charged with recklessly causing grievous bodily harm.

Barry sobbed when he appeared at Parramatta Bail Court on Saturday afternoon. He chose not to apply for bail and will re-appear at Central Local Court on Monday.

The brothers had been out to dinner with their father in Sydney on Friday night before continuing on without him for a drink.

Patrick's friends and family in Ireland had been keeping an affectionate eye on his travels around Australia. After arriving in Sydney, Patrick travelled to Melbourne and started a road trip with mates along the Great Ocean Road on June 7.

By June 18, he was telling friends: "Another road trip to commence yeeooo!! To Adelaide, Alice Springs then Darwin."

Barry appeared the doting older brother as Patrick posted photos of his adventures in Australia.

"Stay away from the edges ya mad man!!" Barry wrote after Patrick posted a photo of him standing on the edge of a cliff in Darwin. "Great pic though!!"

Patrick arrived back in Sydney late last year and was reunited with his brother and father, 74, three days ago.

The brothers, who have four older sisters, appear to be the best of friends in photos on social media. Facebook shows the tight-knit family loved spending time together and the brothers revelled in spending nights on the town in each other's company.

"Bros in arms," their cousin Stephen wrote after Barry posted one snap of the pair together.

The Commander of Kings Cross police, Superintendent Michael Fitzgerald, said the men's father was in shock.

"This is a tragedy," he said. "We have a father who is facing the reality of one son facing serious criminal charges, another son fighting for his life."