Update - 1.45pm: The National Bus and Rail Union has warned of industrial action if more bus routes in Dublin are privatised.
NBRU General Secretary, Dermot O'Leary, said: "Despite the fact that that Dublin Bus (with the vital support of our members) has been hitting all the stringent performance related targets set down by the NTA, this Government has decided to pump significant amounts of taxpayers' money into the bank accounts of multinational operators.
"It is disappointing that Dublin Bus were unsuccessful in the so-called tendering process.
"It would appear that those alleged experts in the NTA and the Department have decided that workers’ rights are up for grabs and can be used, Dutch auction style, to help towards bidding for state contracts. The price of labour should not be used to win such contracts."
Mr O'Leary went on to say that his union would continue to fight for bus workers' rights.
He said: "The NBRU will focus our attention on ensuring that our affected members on the privatised routes will not be forced to move from their current workplace, we will also move to recruit those new entrants that will work for the private operator to ensure that they will be properly represented and work with them towards achieving similar terms and conditions as those workers we represent in the state-owned companies.
"This Tory-lite Government, disgracefully supported by the rag bag Independent Alliance, would be wise to restrict this foray into the privatisation of publicly-owned transport services to the routes identified, moving beyond will not only be vehemently opposed by the NBRU, but will inevitably lead to major industrial unrest across the public transport system."
Update - 12.52pm: The Workers’ Party has condemned the decision to award a licence to run 10% of Dublin’s bus routes to UK bus company Go-Ahead.
The party said it marks a major move towards privatisation of Dublin’s bus services and in making Dublin Bus unviable and ripe for picking off.
Workers’ Party Dublin Councillor Éilis Ryan said the decision was "always inevitable" and claimed the National Transport Authority is "fundamentally and institutionally biased against Dublin Bus and the concept of publicly owned transport".
Cllr Ryan described Go-Ahead as a "predatory operator whose aim is profit and the destruction of its competitors".
She said: "This is a disastrous decision for workers and commuters alike. For workers it represents another step in the ‘race to the bottom’ with lower wages, longer hours and worsening conditions.
"For commuters it means a cherry-picking of services and very probably falling reliability from a company whose railway division was voted ‘worst rail operator’ in the UK in 2015.
"This decision has nothing to do with cost or competitiveness. It is a deeply political decision to begin the process of privatisation of public transport in this country. One only has to look at the result of this privatisation in Go-Ahead’s home country, the United Kingdom, to see that this spells very bad news for consumers, taxpayers and workers."
Update - 11.32am: English firm Go-Ahead has been selected as preferred bidder for 10% of Dublin Bus routes.
The firm is one of the largest bus operators in London, and will start in Dublin at the end of next year.
Only Dublin Bus and Go-Ahead were involved in the final stages of the tendering process after four other firms dropped out.
The National Transport Authority says Go-Ahead offered the best value for money to run the 23 routes and a new route being created.
Earlier: The result of a tendering competition for 10% of Dublin Bus routes will be announced later this morning.
The 23 routes all serve outer suburbs of the greater Dublin area and do not go into the city centre.
They include the 220 from Ballymun to Blanchardstown and the 75, Tallaght to Dun Laoghaire.
It is understood Dublin Bus, Luas operator Transdev and a number of British bus companies are among those who applied for the franchise.
Unions previously objected over fears the move would lead to privatisation.
However, they are not expected to oppose today's announcement after securing guarantees in labour talks.
The results will be announced by the National Transport Authority later.