The pretty teenager, who was due to turn 16 on Christmas Day, took her own life at a house in Donegal on Wednesday night.
Her devastated family are preparing to bury Shannon beside her 13-year-old sister Erin, who also died by suicide on October 27.
Poignantly, Shannon will be laid to rest on the seven-week anniversary of her sister's death.
It will be a traumatic yet fitting gesture for the girls' mother Lorraine, who described the pair as "inseparable".
"Erin and Shannon were the lights of our lives," she said.
"They were as close as any two sisters could be. We are just devastated by what has happened and we just can't say anymore. We are all just numb."
Heartbreaking new details have emerged of the days leading up to Shannon's death.
Just last week, the teenager spoke to her grandfather about her plans to go to university.
She was bravely struggling to repair her life and missed her "wee sister" immensely, according to James Gallagher.
"I spoke to her the other night and she seemed all right. We were making plans. She was going to come and stay with me and talk with me," he told a radio station.
"She wanted to come to Scotland to go to university. She sounded really good. We planned ahead. It's not going to happen now."
But the young girl had left emotional messages on Facebook, conveying her pain.
One message read: "Morning beautiful. Everywhere I look I see that cute smile that I adored. Your wee laugh plays over and over again in my head.
"Still can't come to terms with that I won't see or hear that again. I don't believe you're dead yet.
"I even found myself ringing you last night and then it really hit . . . sleep tight darling."
Choking back tears, Mr Gallagher spoke of how the family is coming to terms with burying a second child in just five weeks.
"I've spoken to Lorraine and she is devastated beyond words. It's really hard to take in and we are just trying to get our heads around it," he said.
"We are now preparing for another funeral and I just don't know what to say."
The community of Stranorlar, where the girls went to school, and their hometown of Ballbofey are filled with grief.
Shannon had been staying at a house in Castlefin, not far from the graveyard where Erin had been laid to rest.
It's understood adults in the house had checked on her just minutes before she was found dead at 10.30pm on Wednesday.
She had spoken about missing her sister and had been dreading Christmas.
Both Shannon and Erin had been students at Finn Valley College in Stranorlar.
Special prayers were led by local curate Father John Joe Duffy as the HSE activated an emergency response plan for the second time in seven weeks.
"The HSE extends its deepest sympathies to the family and friends of the deceased," said a spokeswoman.
"The HSE is liaising with the family in relation to this incident and all supports are being made available to provide assistance at this difficult time."
At the school, parents were stunned by Shannon's death.
"Erin's death was bad enough but now this; it's heartbreaking for everyone in the school," said the mother of one student.
The Chief Executive of the Donegal VEC Shaun Purcell said it was "an extremely difficult time for all concerned".
He added: "Our thoughts and prayers are with Shannon's family at this sad time."
Erin had been tormented by online bullies. Shannon, friends said, had been "the perfect big sister" in trying to help her through the ordeal and was left distraught at her sister's death.
She will be buried alongside her sister in Castlefin cemetery after 11am Requiem Mass in Stranorlar tomorrow.