It happened in Merrion Street, in the city centre, in the early hours of this morning.
Police were called by the ambulance service at about 3.50am to reports of an assault.
They found two men with injuries and they were taken to hospital.
One man was seriously injured.
Three men have been arrested in connection with the incident and they remain in custody.
Police have cordoned off part of Merrion Street, close to St Johns Centre, while they investigate the attack.