Mr Barry, 61 claimed the award for his novel Days Without End
Sebastian Barry has won the Costa Book of the Year award for a second time.
Mr Barry, 61, who won in 2008 with The Secret Scripture, claimed the award for his novel Days Without End, which is set in 1850s America.
The Dublin-born author and playwright, who lives in County Wicklow with his wife and three children, was inspired to write about a gay relationship after his son came out.
Judges' chairman Professor Kate Williams said Mr Barry was the unanimous choice for his 'searing, magnificent and incredibly moving description of how the West was won'.
The book centres on teenager Thomas McNulty and his brother-in-arms John Cole, who fight in the Indian Wars and the Civil War.
Professor Williams noted: 'Sebastian Barry said his son came out and he wanted to write a gay relationship.
'These two men find engagement and Beauty in each other.
'I think that is what is so wonderful about the book - it takes you to the most brutal moments of humanity... and yet you have these moments of love, friendship and redemption.
'It is just such beautiful characterisations and brilliant writing.'
Mr Barry is the first novelist to win the coveted award twice, a feat which has also been achieved by poets Seamus Heaney and Ted