Cork Mental Hospital, also known as Our Lady’s, was once the longest building in Ireland: a monster of 19th-century gothic, much added to before its closure in the 1990s, that stares from the north bank down to the River Lee and the city beyond. In recent years, a lot of the complex has been turned, predictably, into apartments. A developer’s website now invites you to “Live comfortably, live conveniently, live with us”.

This, surely, is a spectral sort of invitation: hard for “us” not to conjure, amid bright mockup interiors, the fretful shades of the unwell – and the unwilling. When Doireann Ní Ghríofa – celebrated poet and author of the nonfiction A Ghost in the Throat – began exploring the derelict site several years ago, she recognised it straight away as a place she might herself, but for historical fortune, have ended up. Said the Dead is an intimately researched but also wildly imaginative study of lives (mostly female) lived and often concluded during the hospital’s first 70 years or so.

The book’s historical span is a matter of official constraint. When she goes divining in the archive, chiefly in the hospital’s large green casebooks, Ní Ghríofa must stop reading at a century’s distance: anything more recent risks breaching confidentiality. As a result, the Victorian and Edwardian voices she has been hearing fall silent in the early years of an independent Ireland.

Regardless, her notes seethe with the names, characters, adventures and misfortunes of patients. Bridget, heavily pregnant, who had emigrated to America but was thrown out and sent home by her brother when he discovered her condition. Anna Martha, a painter, “peculiar in her antics”, who pulled a gun on magistrates who wished to put her in the asylum. Sixteen-year-old Dora, who “wishes to be dead”: a great reader of novels, beaten into depression by her parents. Muriel, whose husband was Terence MacSwiney: republican lord mayor of Cork, soon to die on hunger strike in Brixton prison.

There are names that fade quickly from the record, others that stubbornly, mysteriously or even merrily persist in the archival pages. Behind these accounts of lives ruined and sometimes recovered, there are the doctors who treated the women. In the archive, their voices are most forthcoming at the time of admission, recording fears and delusions. “Says that fairies work on her nerves … Said she has changed into many shapes since I last saw her. Said that she will be burned soon, and that people are foretelling it.” Affect and intellect are noted: “dull”, “sullen”, “stupid”, “intelligent”. In many instances, these accounts decline into seemingly careless repetition: “No change.”

But in 1896, into this institution arrived Lucia Strangman, the first woman qualified as a psychiatrist in the British Isles. She is Ní Ghríofa’s double in Said the Dead, a reader of faces and bodies and letters, a listener to voices on the edge of extinction. On the evidence here, Lucia seems to have been at the humane, inquiring end of early 20th-century psychiatry.

Reading is Ní Ghríofa’s version of doing justice to these lives, but reading is double-edged, a kind of love and a type of surveillance. Early on, her presence on the page sunders: she is there as an exploring “I”, but refers to herself some of the time as “the Reader” who presides even over Lucia and her staff, who assumes authority and responsibility for all of these dead, vivid souls. Ní Ghríofa’s treatment of the patients and their textual remains is never less than sensitive. Like Freud with certain celebrated cases, she will use first names only. But the Reader is also obsessive and susceptible: she is the one who pursues the dead, impossibly, out of the written record and into their hopes and regrets, dreams and extravagant desires. It is these that give this book its extraordinary formal and ethical force.