The cover of Sinéad Morrissey's excellent TS Eliot prize-shortlisted collection, Through the Square Window, shows a young girl in the head-bowed, arms-raised, slightly knock-kneed posture of a preacher channelling the word of the Lord. It's an unsettling image that resolves itself into a picture of innocence when we read the title of the photograph on the back cover: Girl About to Do a Handstand (1957). To have been wrong-footed makes a fitting first impression, for the book's central theme is early childhood: a time when expectations will be flouted and outflanked. In 'Cathedral' the speaker addresses the infant, saying 'I wanted the words / you attempted first to be solid and obvious: / apple, finger, spoon'. But language will not be bidden or held back, and the two-year-old surprises his parents by announcing: 'at six o'clock the ghost / of a child might come and eat porridge. / We are speechless.'

Morrissey is too clear-eyed to allow anything whimsical to encroach on her observations: a baby is born 'crook-shouldered, blue, believable, beyond me – / in a thunder of blood, in a flood-plain of intimate stains'. Nevertheless, the experience of motherhood returns the poet to something like a child's perspective, and many of the poems strike a tone of hushed excitement. This is particularly effective in the poems about situations that bring out the inner child in all of us, such as listening to a thunderstorm at night, brilliantly evoked in the opening poem, 'Storm'.

Childhood brings with it a host of real and imagined dangers, and sometimes the source of the danger turns out to be the child itself. 'The Innocents' engages with the 1961 film of that name and with Henry James's The Turn of the Screw, ending with the image of 'Master Miles / . . . considering his goodnight kiss'; and in 'Telegraph' we follow a child through the depressingly familiar arc from abused to abuser. The poem is a sestina, and the repeated end words tell much of the story: child, house, witness, window, night, fault. The poem cannot escape these end-words any more than the child can outrun his circumstances: 'Whose fault that for twelve years afterwards in that house / a man slipped into the room of a child, kept back from the tiny window, / and nightly undid what only the hawk moths witnessed?'

The title poem is a dream vision set in l'heure bleue. In an unnerving matter-of-fact tone we are told that the dead have 'arrived / to wash the windows of my house'; though the speaker senses that they have really come for her son, who 'sleeps on unregarded in his cot'. The poem subtly draws together the book's motifs – children and the dead, clouds and water, windows and witnesses – while the observing eye remains as precise as ever: 'The clouds above the Lough are stacked / like the clouds are stacked above Delft. / They have the glutted look of clouds over water.'

The reference to Delft evokes the distinctive blue porcelain named after the city, and also Derek Mahon's poem of imagined childhood, 'Courtyards in Delft'. When one of the dead appears as a 'blue boy', we recognise another suppressed allusion, this time to EE Cummings's enigmatic question 'how do you like your blue-eyed boy / Mister Death'. The poem is concerned with inheritance, both in terms of literary precursors and the world that awaits the sleeping child. The anxiety that the child may be threatened by the dead asks to be read in terms of Morrissey's Belfast upbringing. Suddenly, the speaker wakes from her vision: 'flat on my back with a cork / in my mouth, bottle-stoppered, in fact, / like a herbalist's cure for dropsy.  'Dropsy is an excess of water, returning us to the image of those ominous, glutted clouds: the speaker has absorbed the poem's dangers, in an attempt to protect the child from them.

Along with Colette Bryce and Leontia Flynn, Morrissey is one of a number of younger poets from Northern Ireland who are negotiating the mixed blessing of having such illustrious antecedents as Mahon (also a cloud-watcher poet) and Seamus Heaney ('The Invitation' features a child-Narcissus, as in Heaney's celebrated 'Personal Helicon'). To honour such an inheritance requires all the confidence and care of a high-wire act, and Through the Square Window shows Morrissey is more than up to the task. With its incisive imagery and taut rhythms, the collection is a formal triumph; but what makes it truly marvellous is the emotional pressure Morrissey maintains: the poems come to us with the intimacy of whispered secrets.