People read aloud names of children as they gather at Tuam (Niall Carson/PA)
Ms McKay (64) knew nothing of her oldest sister, Mary Margaret O’Connor, for years.
The child is reported to have died in the mother and baby home in 1943 from natural causes.
“People are shouting to have this place dug up, it’s an obscenity,” she said.
“We could never imagine there would be a septic tank with almost 800 children in it, never in your wildest nightmares.
“I don’t know a country that would put 796 babies in a disused sewage tank.
“I look at the Pope (Francis) coming to Ireland and then I hear people talking about the last Pope’s visit here in 1979 and how they loved him and the Catholic Church but time and time again I hear about abuse and scandal.
“We are telling the church you must change.
“The Pope and the rest of them should have come here and listened to what the Catholic Church did to 796 children.”
Ms McKay, who lives in Manchester, and many others are calling for the site to be excavated and for the identities of the remains to be made known.
“Those babies aren’t hidden, they are in that septic tank and they need to be given back to their families,” she added.
In 2013, local historian Catherine Corless discovered official records showing that around 800 children died at the home in Tuam.
Hotel bedsheets with the names of hundreds of dead children draped on the gates of a mass burial site at Tuam, Co Galway (Niall Carson/PA)
A special sculpture of babies in the shape of a baptismal font, which was made by a Belgium woman, took centre place in the art work.
The artwork sat at the spot which would have been the back of the old mother and baby home.
Only one wall remains of the baby home which is a monument to six men who were shot in 1922.
Ms Corless, who carried a small makeshift coffin to the site, said: “It’s important that we give them a funeral to let the babies know we are still here as I believe their spirits are here and they are calling out to us.
“They deserve to be exhumed and buried. It was horrible what they did to them.”
Reacting to the large turnout, she said: “I can’t put words to it.
“This is to make a statement and it’s overwhelming to know there is great support for the Tuam babies.