A portion of the tri-colour, which was flown at Jacob's Factory in Dublin during the Easter Rising, has been presented by a Jersey family to Glasnevin Museum.
The pieces of the flag were given to the museum by the grandson of a member of the Jacob's Garrison in 1916, who only recently realised their historical significance after reading about the flag in his local paper.
David Blake's grandfather, John le Provost, was among three men who made the Jacob's factory tri-colour.
Le Provost posted three pieces of the a flag - along with a letter - to his future wife, Mary La Singe, who was living in Jersey.
Speaking at the presentation ceremony in Glasnevin this afternoon, David Blake explained how he found out through his local paper that his family's heirloom was of huge historical importance.
"A footnote asked that if anyone has any further information, could they contact the paper," he said.
"I did, and I explained that not only was the extract taken from a letter written to my grandmother, but I still had the original letter and the flag.
"I do not think Usain Bolt would have beaten them to my door."
He added: "It is only correct and necessary that the Jacob’s Tricolour comes back home.
"I feel as if I had it for a reason; as if the family had been caring for the flag so when the appropriate time came, it should go back.
"This year is the appropriate time."
Chairman of Glasnevin Trust, John Green, explained the importance of the flag, and expressed his thanks to the Blake family: "This is a special day for Glasnevin Cemetery Museum; the presentation of the largest known remnant of the Jacob's factory Tricolour.
"It is not only the symbolism of the flag returning to Dublin, to the final resting place of its makers but also the stories it enables us to tell.
"The story of the Jersey contingent and the Royal Irish Rifles; the story of how the flag went to Jersey.
"The extraordinary letter of provenance from John Le Provost which complements the witness statement of Thomas Meldon so well.
"Glasnevin has several other links with the Jacob's factory and the Flags of 1916. Through the exhibition of this flag we will be able encourage museum visitors to delve deeper into this complex period in our history.
"Thank you to David Blake and his family for presenting it to us but most importantly for preserving it over the years."