The discovery was made during the lockdown when staff at the museum were cataloguing a collection of artefacts and papers on loan to the museum from the Collins Family.
A small booklet belonging to Collins was opened and a small envelope fell out with 'M A O'Brien Collins' and 'Mothers funeral + mass card' handwritten on it.
Inside, was a memorial card to Marianne O'Brien, Michael Collins' mother who died in April 1907.
"Interestingly her age at her death was incorrect and underneath '54' someone had written 52.
"This struck a chord with our staff and we realised that this card was the one described in the Michael Collins biography by Tim Pat Coogan.
"The numbers '52' had been written by an 'ever sentimental, ever meticulous', 17-year-old Michael Collins," explained Jamie Murphy, Manager at Michael Collins House.
"Michael Collins is often referred to as the 'Big Fella' and 'The Man Who Won the War'.
"This little envelope that Michael Collins held so dear shows a different side to Collins, a more fragile one.
"Then just a 17-year-old who had just lost his mother, and indeed both his parents at this point, while he attempted to begin a new life in London," Mr Murphy told.
Along with the memorial card and envelope, staff at the museum also found a much-handled newspaper clipping of a eulogy and poem in memory of Marianne which was printed in the, Michael Collins' brother-in-law's paper.
The memorial card and envelope are now on display at Michael Collins House.
Since reopening the museum is now operating on a pre-booking system.
To book a slot, you can phone 023-8858676 or email email@example.com.