The sad news was revealed in an emailed newsletter to journalists from Áras an Uachtaráin.
Síoda had been recovering after an operation earlier this year.
Síoda will be sadly missed by Bród - the Higgins family's second dog - as well as by the President, his extended family, and the whole of Ireland.
Bród and Síoda have been features of the Áras since President Higgins took office, welcoming visitors and were often seen going on walks in the Phoenix Park with the President.
People have taken to Twitter to express their sympathies and fondness for Síoda.
— Shannon Forrest (@waittilitellya) September 19, 2020
The President has often spoken about the popularity of his two dogs, saying in 2018 that guests would ask to meet them.
"There was a group in to watch the credential ceremony from Loughrea ICA and as I had all but said goodbye to them, they said 'but we haven't seen the dogs yet'," said President Higgins.