Candice (28) hasn't seen her daughter since she waved goodbye at Dublin Airport in July and she now fears something may have happened to her.
Ms Gannon's former partner, Filipe de Silva (35), failed to return the little girl to her mum after the holiday in the Algarve in July.
She said that the last week has been a "nightmare" for the family without her "Christmas baby".
But she vowed to stay strong for the sake of her youngest daughter, Olivia (2).
The distraught mum said that she and her ex used to take turns around Christmas so that they could both see their daughter.
But Candice planned to stay in Ireland throughout the festive season with her husband Philip and Olivia.
The family split their time between Dublin and Madeira and plan to return to Portugal in the early new year to search for Ellie.
"I'll be spending Christmas here. I have a big support network here -- a lot of friends and family," she told the Herald.
"I am a lot happier being back. I miss bringing my little girl to see Father Christmas and all the traditions that are here.
"We are definitely going back over to see if we can find out anything else in the new year.
"I doubt that she is enjoying her eighth birthday. The little girl's life was taken away from her; she was ripped away from friends, sister, stepfather and her life."
Ellie has a British passport. "I have been in touch with the UK border control and I will notify the police in the UK," Candice said.
"The hardest thing is not knowing from day to day."
Filipe de Silva broke a court order when he failed to send Ellie back home.
Candice, who is originally from Cheltenham in the UK but has an Irish passport through her Ireland-born grandparents, said that it is particularly difficult at this time of year.
She keeps going for the sake of her youngest daughter. "I am so stressed out, I have to keep this up, keep strong," she said.
"I just keep up the beliefs, knowing that I will see her.
"All I can rely on is having the public looking out for her."
Further information is available at www.reuniteellie.com