During the horror ordeal, the tourist had his head split open with a crutch while his male friend suffered a broken nose and fractured cheek bone as the gang punched and kicked him in the head.
One of the gang was arrested after he was caught by a garda who was near the scene, while the other two escaped.
The Californian man, aged in his early 30s, and his two Dublin friends -- a man and a woman aged in their late 20s -- were attacked by the crazed trio as they sat in the park in Merrion Square on Tuesday evening.
During the ordeal, the tourist had his "head split open" with a crutch while his Irish male pal suffered a broken nose and fractured cheek bone as the gang punched and kicked him in the head.
The woman was also assaulted and mobile phones, wallets and cash belonging to the three friends were stolen by the gang members, who are originally from south Dublin.
Sources say that during the prolonged assault, the thugs made threats to kill the pals, who had been enjoying a relaxing evening in the park before they were attacked, at around 8pm.
The gang fled the scene after the attack, but a uniformed garda from Pearse Street Station who was in the area saw what had happened and arrested one of the suspects.
The other two escaped before back-up was called, but officers are satisfied that they know their identities and they are expected to be arrested in the coming days.
A senior source said: "What happened in Merrion Square on Tuesday night is absolutely horrendous, but the sad reality is that this kind of thing is happening every day now.
"There has been a major increase in this type of crime in recent months especially in Dublin. The two men who were attacked required hospital treatment and all three victims are extremely traumatised about their ordeal.
"The garda who made the arrest should be commended for his quick thinking at the scene."
The Herald can reveal that the arrested man is a 21-year-old from Rathfarnham who is well known to gardai.
He has a number of previous convictions including for possession of a knuckleduster.
The suspect received a four-year suspended sentence for his role in the aggravated robbery of a south Dublin shop in which staff were terrorised.
On that occasion, the suspect and two of his associates entered the shop armed with knives and hammers and stole cash and cigarettes.
The suspect is expected to be charged with multiple offences later.
In a sign of the violence that is engulfing our streets, the Merrion Square incident happened just hours after an 86-year-old woman was mugged on her way home from a bank in Kilkenny city.
Mary Nolan, who uses a crutch, was knocked to the ground by the attacker, who then snatched her purse containing €200 in cash.
Shortly after the attack, gardai detained a 35-year-old Polish man and recovered the money.
A major garda investigation is now under way after the condition of the victim deteriorated last night.