He received his secondary education at Templeogue College; he recalls the subjects as being "horrifying," with the exception of art classes with a favourite teacher, Mr Weafer.

[4] He applied to the College of Amenity Horticulture at the Botanic Gardens in Glasnevin, Dublin, failing the first time, but securing entry three years later.

[2] Gavin started work with small jobs, the first, a suburban house garden featuring gravel and railway sleepers, in 1988, while he was studying at Glasnevin, still intact as of 2016.

[5] A notable early commission was for social columnist Terry Keane, a contract during which he also met his future wife.

[2] Gavin runs his own garden and landscape design business, with an office in London, and contact details in Ireland also.

[1] In particular the projects in China have tended to be large-scale and of long duration, involving a lot of time away from home, and limiting his work in Ireland and the UK.

By the end of 2019, Gavin has participated in the Royal Horticultural Society's Chelsea Flower Show nine times, seven on the main avenue, the last having been 2016.

[6] Gavin described the garden as inspired by the Oscar-winning animator Richie Baneham (who drove visual effects for Avatar), as well as by the "suspended table" novelty restaurant concept, and by the Midsummer Festival of the Senses at Cork, the landscapes of Capability Brown, Charles Jencks and the some Italian Renaissance figures.

[citation needed] Gavin has also participated in promotional work for Ireland's horticultural development board, Bord Glas, free of charge.

On 12 December 2010, Gavin appeared on Channel 4 on a Christmas edition of Come Dine with Me with David Gest, Hannah Waterman and Sherrie Hewson.

In 2007, Gavin received an Honorary Degree of Doctor of Art from Nottingham Trent University in recognition of his international reputation for contemporary garden designs.

The Gavins live, and Diarmuid manages their garden, near the village of Kilmacanogue, south of Dublin, in County Wicklow, Ireland.

On the one third of an acre around the house, which he enhanced with beams from the former Jervis Street Hospital, he has ponds, a shed created from his Dundrum Town Centre pop-up shop, and items from some of his Chelsea Flower Show gardens.