Officers investigating the murder of Mary-Ann Leneghan, whose family is from Co Mayo, arrested a man in his 20s today, Thames Valley Police said.
The suspect, who was arrested in London, is the fifth person to be held as part of the inquiry.
Shocked family members in Mount Jubilee, Bangor Erris were being comforted by neighbours after news broke of the murder, whose father Bertie emigrated to England in 1972.
Sixteen-year-old Mary-Ann, from Reading, Berkshire, died from a stab wound to the neck, and her friend, who cannot be identified, is in hospital after being shot and stabbed.
Both teenagers were sexually and physically assaulted by a gang of at least six men.
The young women suffered a “disgusting ordeal” lasting several hours at the hands of up to six attackers in the early hours of Saturday, police said.
The second girl is a key witness and is helping police with their inquiries from her hospital bed where she is in a serious but stable condition.
Superintendent Dilip Amin, police area commander for Reading, said it was “very fortunate she was still alive“.
He said: “She is very ill from the injuries she has received, but she has been very brave in helping police.”
Police believe the two young women may have been deliberately targeted, though they were not able to confirm whether the attackers were known to their victims.
They were finally left at a remote spot in Prospect Park in Tilehurst, Reading. The 18-year-old managed to make her way to a nearby road and flag down a passing motorist who rang for help.
Mary-Ann’s body was discovered in the park by police at 5.30am.
Detectives arrested another man in his 20s yesterday at a south London address but he was later released on bail until June 2 by Thames Valley Police.
Mr Amin said last night: “Information we have suggests that this is not a random attack.
“We are keeping all our options open as to what the motive was.“
A Thames Valley Police spokeswoman confirmed that both girls are thought to have been sexually assaulted.
The pair’s ordeal began between 10pm and 11pm on Friday evening when they were approached by a group of men in a car park near a derelict pub in Caroline Street, Reading.
They were taken from a white car to a maroon car, which was parked in Wickes car park on Weldale Street, and arrived at the Abbey Guest House on Connaught Road in the red light district of Reading at 1am.
The vehicle, being driven erratically and at speed with loud music blaring, left the guest house between 3am and 5am.
CCTV footage is being scrutinised in an effort to trace the vehicle.
Officers believe that it was at the guest house that the girls were subjected to numerous “horrific and terrifying” assaults.
The family and friends of Mary-Ann have been laying tributes at the spot where her body was found.
Her father Bertie laid flowers at the scene. Her mother, Sue, also left a floral tribute with a note that read: “Hope you are at peace, my darling. I love you and miss you babe.”
Another tribute from a school friend read: “You will always be in our hearts and our thoughts.“
Grieving pupils from nearby Prospect College, which was attended by Mary-Ann, placed their school ties on top of the floral tributes.
Prospect College said in a statement: “We are deeply saddened by the tragic news of the death of Mary-Ann Leneghan.
“Mary-Ann was a year 11 student at the college. She was a popular and friendly student and was due to sit her GCSEs in a few days’ time.”
Martin Salter, Labour MP for Reading West, branded her death an “absolutely horrific murder“.
But he assured his constituents they could still feel safe to walk the streets. He said: “We have not got people hiding in the hedgerows.“
A £15,000 reward is being offered for information leading to the arrest and conviction of the killers.