The Alternatives begins with a lecture on geological destruction. Olwen Flattery, a thirtysomething academic with a razor-sharp wit and a soft spot for hard liquor, tries to impress on a roomful of Galway undergraduates the aeons of tectonic convergence it took for the Earth to reach its current form, versus the terrifying speed at which we “needy, grabby, arsony narcissists” seem bent on destroying it.

Olwen soon grows frustrated by the confines of the lecture theatre, the inability of words to convey her urgent message. She mounts her bike and leads an unofficial field trip to the Atlantic coast, where she lets the rocks do the talking. And that night, after dinner with her partner (and a few hefty gins), Olwen mounts her bike again and cycles out of her home, out of the county, out of her entire life.

Unfortunately for her, instead of aeons of solitude, Olwen manages only a few months before her new life is disturbed by a trio of uninvited guests. For as well as being a geologist and a high-functioning alcoholic, Olwen is also the eldest of four sisters who were orphaned in their youth. There is Maeve, the celebrity Instagram chef, now living on a houseboat in London with a mime artist she may or may not be in love with; there is Rhona, the Trinity academic who lives in a “well-heeled enclave” of Dublin with her baby son and Peruvian postdoc-cum-nanny; and there is Nell, the adjunct philosopher, in the US with no healthcare (and thus no means of investigating the chronic numbness slowly consuming her lower limbs).

If not exactly estranged, the Flattery sisters haven’t been together for years; Olwen’s escapades serve as a catalyst for reunion. So they come together in the wilds of County Leitrim to rake over the past and to think about the future – their own and that of the planet.

Hughes has written about sibling dynamics before. In her debut, Orchid and the Wasp, the ruthless Gael Foess was counterbalanced by her vulnerable baby brother Guthrie. Her follow-up, The Wild Laughter, centred on the Black brothers in the face of their farm’s – and the entire country’s – collapse. The Alternatives is Hughes’s best novel yet. With its ferocious intelligence and furious wit, the writing is reminiscent of Anne Enright. The structure too recalls Enright’s The Green Road, as we spend time with each sibling in turn, observing the particular texture of their individual worlds, before they find themselves together in the same room (or, in this case, the same County Leitrim field).

At this point, Rhona drolly remarks that it’s “curtains up” for the psychodrama – a remark to be taken literally, since the form now shifts, tectonically, from prose to playscript. Using dialogue and stage directions, Hughes brings the reunion scenes to life, the effect less Enright and more Brian Friel’s Dancing at Lughnasa.

Like the Mundy sisters of Friel’s play, the Flattery sisters share a steady patter of communal banter while each harbouring their own private concerns. They also recite Emily Dickinson poems, quiz each other on Heidegger, debate the difference between eco-Marxists and eco-socialists and argue over how long Rhona’s Tesla will take to charge via Olwen’s makeshift generator.

Considering the exceptional energy when they are together, it is a shame it takes so long to get there (the sisters don’t cross paths until over a third of the way through the novel). Meanwhile, given the proliferation of metaphors, some are more convincing than others. An exchange student asking a question “with the gall of a parking enforcement officer” works well; a baby and an adult “inspecting each other like tourists considering the admissions fee to a museum of lesser-known history” feels more forced.

Overall, though, The Alternatives is a brilliant, brainy book about the bravery of following one’s own path while also remembering the value of community. It is about the fundamental unknowability of people, even those closest to you – unlike rocks, you “can’t dig a borewell into them. Find their groundwater.” But it stresses the importance of coming together anyway – to laugh, to eat, to try.

• The Alternatives by Caoilinn Hughes is published by Oneworld (£18.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply.