The tragic story, which has laid hidden for some 90 years, will be revealed during the launch of a book, In Search of Michael Collins, by Tim Crowley.

Mr Crowley is curator and founder of the Michael Collins museum and interpretative centre at Castleview, , Clonakilty, where the book is launched tomorrow at 7pm.

This will form part of a special double launch, as the event will also feature copies of a fascinating journal written by Mr Crowley’s great-grandmother, Marianne Slyne, née McCarthy, who was a second cousin to Michael Collins’s mother.

In the journal, which dates from the 1870s, Ms Slyne makes several references to the family of Michael Collins — in fact the very last entry summarises the events surrounding the death of Collins at Béal na Blath on August 22, 1922.

“The big puzzle was always why Marianne Slyne stopped writing her journal following that last entry about the death of Michael Collins,” said Mr Crowley. “We have since discovered that it was the subject of a family secret.”

One night in 1922 or 1923, a band of anti-Treaty IRA fighters entered Ms Slyne’s farmhouse kitchen near Enniskeane, demanding a horse from her stable.

Being on the other side of the Civil War divide, she refused and, angered by her refusal, one of the IRA men fired a shot through the ceiling — straight into the bedroom upstairs where Ms Slyne’s daughter, Elizabeth, was lying in bed.

Initially Ms Slyne believed her daughter — who would later become Mr Crowley’s grandmother — had been killed. Although this soon proved not to be the case, the invasion into her home upset her to such an extent she suffered a nervous breakdown and never again wrote in her diary.

“I am involved in history all my life and I never knew this until recently — at that time,” said Mr Crowley, who had the journal scanned and published.

“Any form of mental illness was always hidden away, and this is why we never knew until lately what happened.”

A limited number of copies will go on display at the centre at the launch, alongside copies of Mr Crowley’s book, which is self-published and will be launched by Collins’s grand-nephew, Maurice Collins.