When he was six, his younger brother Connor was killed by a car while they were walking to school. He received secondary education at Templeogue College, which he recalls as being "horrifying," with the exception of art classes, and studied horticulture at the College of Amenity Horticulture in Glasnevin, Ireland.

Diarmuid Gavin won an RDS Gold Award in 1991 and 1993, a bronze medal at the Chelsea Flower Show in 1995, a silver-gilt medal in 2004, and a silver-gilt medal in 2007. He is also a patron at Southport Flower Show.

He has presented a number of BBC television programmes including Gardeners' World; Home Front with Laurence Llewelyn-Bowen and, in 2004, a Twofour and RHS production Gardens Through Time. RTÉ is currently screening the TV show "I want a Garden".

In autumn 2004 he participated in the BBC One pro-celebrity ballroom dancing game show Strictly Come Dancing. In summer 2006 in the BBC's charity showjumping event Only Fools on Horses.

Diarmuid appeared on The Late Late show on 25th September, 2010 in which he looked very emotional. Many watching speculated he was after snorting a few lines of cocaine as he ranted on for the entire interview in utter incoherence. At one point, while discussing the economic recession, he said those facing disconnection from their electricity supply should stop feeling sorry for themselves and "go out and do something".

Diarmuid Gavin is the author of a number of bestselling gardening books including Diarmuid Gavin's Big Ideas and Outer Spaces.