Shannon was found dead by the foster family she was staying with as she tried to come to terms with the death of her 13-year-old sister, Erin, who took her own life less than two months ago.
Yesterday, at her funeral Mass, Father John Joe Duffy said the systems and methodologies put in place by the "HSE and other agencies" for dealing with youth mental health "simply does not work".
"Society has failed two children, two sisters within two months and that is a most damning indictment for any society evidenced by the second white coffin that is before this altar," he told the congregation in an emotional address.
Erin was found dead at her family home in Ballybofey, Co Donegal, on October 27 and online taunts and schoolyard bullying are alleged to have played a role in her death.
Last Wednesday, while wrapping Christmas presents, Shannon posted a message on Facebook addressed to her little sister saying: "This is the hardest thing I've had to do baby doll."
She was found dead several hours later at the foster home she was staying in on the Lifford Road in Castlefin, about 10 miles from her family home.
Comforted by her father, Seamus, Lorraine Gallagher and her four-year-old son, Sean James, along with extended family and friends said an emotional goodbye to Shannon at Saint Mary's Church in Stranorlar.
Addressing the congregation, Fr Duffy said society failed Shannon and Erin and is continuing to fail the most vulnerable children in our country.
"We are failing to bring in the children, young people and vulnerable adults on the margins of our society to the safety of the centre where they ought to find support, comfort and safety," he said.
"The system and approach needs to be radically improved or to be completely overhauled to be more responsive so they have a relevance and are fit for purpose in a modern world."
He reminded mourners how at Erin's funeral he had asked that no mother should have to experience the trauma of losing a child in the same way that Lorraine had.
Fr Duffy said: "I said at Erin's funeral that no mother in this country should ever again have to experience the acute pain and suffering that you were put through in those days. That was also your wish, Lorraine. But it is most regrettable that you find yourself here once again and so soon in this most tragic circumstance."
During an emotional and hard-hitting 30-minute homily, Fr Duffy said the "time for talking was over" and called for leadership in Government to make reforms.
He said: "Hiding behind professionalism and qualifications or the inertia of being able to change rules, legislation or regulation is simply not acceptable when so many lives have been tragically and needlessly lost. I'm calling for someone in Government to take a hands-on role and to do what it takes, whatever that is, to combat what is the most serious problem affecting our country and so many families and communities with such devastating consequences. The time for talking is over. For the sake of all people on the margins, for the love of God let one person in Government take leadership and reform what is clearly a broken system."
The congregation were told of the strong bond between the two sisters and how Erin's death had caused Shannon "great pain and sadness".
Shannon was also an accomplished athlete and won many competitions in school sports events. She played Gaelic football, soccer and rugby but boxing was her driving passion.
The mourners heard how Shannon had transferred the talents she learned as an Irish dancer to the ring when she took up boxing in recent years. She won county and Ulster titles as an amateur boxer. Members of the Twins Town Boxing Club carried her coffin into Saint Mary's Church.
Fr Duffy also pleaded with the young people to speak about their problems and to seek help if they were having suicidal thoughts
He said: "You may think no one understands you or cares about what you are going through. If you have ever thought of self-harm in any way or ever thought of suicide, then I'm pleading with you to please talk to someone. I don't know anyone who can read our minds. Parents, teachers and friends think they know what's going on in people's lives but they can't really know unless you tell them."