The schoolboy was stabbed on his way home from school in Drogheda.
An incident room has been set up and an appeal for witnesses has been made.
The 16-year-old was on his way home from school at about a quarter past four yesterday afternoon when the attack, which is believed to have been unprovoked, happened on Laurence St in the town.
He was rushed to hospital by ambulance, and is being treated for injuries - which are described as serious - there at Our Lady of Lourdes Hospital.
A man in his 20s was arrested - he's being held under section 4 of the Criminal Justice Act, 1984, at Drogheda Garda Station.
An incident room has been set up at the station and investigating Gardai are appealing for witnesses.
Anyone with any information is asked to contact Drogheda Garda Station 041-9874200, or the Garda Confidential Line 1800 666 111.