Ulster Rugby will be "a poorer place" without Nevin Spence, the team's director David Humphreys has said.
Spence, 22, was killed along with his brother Graham and father Noel in a slurry tank accident at the family farm near Hillsborough on Saturday.
Nevin played centre for the provincial side and represented Ireland in a game against the Barbarians last May.
His sister Emma is in a stable condition in hospital after the accident.
Graham Spence, 30, was a married father of two. Noel Spence, 52, is survived by a wife and two daughters.
Ambulance crews were called to the farm just after 18:00 BST on Saturday.
BBC Ireland Correspondent Mark Simpson said: "Initial reports suggested Nevin's father fell into the slurry tank.
"It was reported that one of his sons tried to rescue him, then Nevin got involved, and his sister Emma."
The Health and Safety Executive (HSENI) said two of its senior inspectors attended the scene on Saturday evening.
"From HSENI's initial investigations, it understands that the three family members who entered an underground slurry tank died from the effects of exposure to slurry gases," it said.
"The exact sequence of events is not yet clear but HSENI is investigating a definite line of enquiry."
Emma Spence was taken to the Royal Victoria Hospital in Belfast where she is recovering from the effects of fume inhalation.
Prayers have been said on Sunday for the family at Ballynahinch Baptist Church, which the Spences attended.
Rev Rodney Stout said the three men were all "incredibly gentle, decent, hard working, with a deep sense of family loyalty".
He said he visited the Spence home on Saturday night and the family were just beginning to come to terms with what had happened.
He said the tragedy would have a huge effect on a very tight-knit community.
Ulster Rugby's Mr Humphreys said Nevin Spence "was a dream player to work with. He was the epitome of what we were looking for.
"On days when nobody else wanted to come in and train, he was there leading from the front.
"All he wanted to do for the remainder of his career was to stay here, win trophies and be successful for Ulster."
Shane Logan, chief executive of Ulster Rugby, said Nevin was "an exceptional young man".
"A man who was killed in his prime aged 22 with an immense future in front of him and we know that because in his 22 years he was a model professional and a model individual," he said.
"A man of absolute integrity, complete determination, great humility, a fine team player and an all-round exceptional individual."
Flowers, scarves and other tributes are being left at Ulster's Ravenhill ground in memory of Nevin Spence.
A book of condolence is to open there at 10:00 BST on Monday.
It will be available to sign until the end of the week and Ravenhill will be open all week from 09:00 BST until 17:00 BST for anyone wishing to leave tributes.
'Outstanding young player'
Billy Glynn, president of the Irish Rugby Football Union, said: "This is a terrible tragedy that has struck the Spence family and the thoughts of everybody in Irish Rugby are with them at this time.
"Nevin was an outstanding young player which was evident from his progression through the representative ranks and the game has been robbed of a very talented young man.
"The rugby community in Ireland is a very close knit one and there has been widespread shock and an outpouring of grief at this news."
Spence had established himself as a regular in the Ulster squad in recent seasons, playing 42 times.
The Ulster centre also came on as a replacement for Ireland in the game against the Barbarians at Kingsholm last May in addition to playing for the Irish Wolfhounds, the country's second-string team, on three occasions.
The former Wallace High School player made his first appearance for Ulster against Ospreys in April 2010.
After an injury-hit start to this season, he played for the Ulster Ravens, the club's reserve side, against Munster A at Deramore Park in Belfast on Friday.
Spence was named Young Player of the Year at the Irish Rugby Union Players' Association Awards last year.
Away from rugby, Spence helped out on the family farm and regularly and enthusiastically updated his many followers on Twitter and Facebook with pictures from the farm.
With his farming background, he recently signed as a brand ambassador for the Dairy Council of Northern Ireland.
The Northern Ireland Ambulance Service (NIAS) sent rapid response teams when it was called to the farm.
Counterparts from the fire service pulled four people from the slurry pit, three of whom were unconscious.
Paramedics tried to resuscitate the three men but two died at the scene. The third man initially responded to resuscitation and was taken to the nearby Lagan Valley Hospital in Lisburn, where he died a short time later.
Health Minister and farmer Edwin Poots, who has known the Spence family for over 30 years, said they were held in high regard in the entire community.
"While a lot of people may focus on Nevin because he played for the Ulster rugby team we do need to focus on Noel and Graham at this time," he said.
"Noel has left a wife and two daughters, Graham a wife and two children. Whilst a place in a rugby team can be filled a place in the home of all three men will never be filled."