For over thirty years, David McDonald worked in Ireland's three biggest prisons. This is his story.

As a young officer, McDonald got a baptism by fire when he started in Mountjoy in 1989. It was a chaotic, smelly and overcrowded place where, inevitably, the approach to keeping order was primitive.

Returning to his hometown two years later, he entered Portlaoise, then Europe's most secure prison due to the presence of IRA and other subversive prisoners. Portlaoise and the Midlands Prison, where he moved in 2000, were also where Ireland's emerging class of serious gangsters were housed and McDonald had daily dealings with household names such as John Gilligan, Christy Kinahan, Brian Meehan, Dessie O'Hare and, latterly, killers like Graham Dwyer. McDonald also shares vivid accounts of the never-ending war of attrition with prisoners over the traffic in contraband like drugs and phones.

Though he rose through the ranks, in time McDonald came to question aspects of what is often a brutal and under-resourced system - concerns that led to him becoming a whistle-blower.

The boredom, the tension, the constant fear, the occasional flashpoints of extreme violence, the moments of comedy, of tragedy and of surprising grace are all captured here with extraordinary honesty and self-knowledge. Written with the help of award-winning journalist and author Mick Clifford, Unlocked is a jaw-dropping and authentic account of life in the toughest of workplaces.