Gardaí confirmed the man in his 40s was arrested and taken to hospital following the incident at about noon.
The man was later taken to Pearse St Garda Station where he was detained under Section 4 of the Criminal Justice Act.
The 1874 oil painting, entitled Argenteuil Basin with a Single Sailboat, is worth millions of euro and is the only Monet in the gallery’s collection.
It was bequeathed to the gallery in 1924 and had been hanging in its Millennium wing.
Gallery director Sean Rainbird described the incident as “shocking and very regrettable”. He praised the gardaí and gallery staff in dealing promptly with the matter and said an internal investigation would take place.
A spokesperson for the gallery described the incident as “unique”.
It is likely, however, that security and public access to the collection will be reviewed following the incident.
James O’Halloran, the managing director of Adams, said the damaged Monet was an “absolutely incredible picture” from one of the most popular periods of French art.
Asked to estimate the value of the painting, Mr O’Halloran said he would not be qualified to put a figure on a painting of that calibre. “Suffice to say it is worth a lot of money,” he said.
Mr O’Halloran said he very much regretted the random attack on such a beautiful picture.
“It is not like it even has a political background to it — it is just an incredibly pleasing landscape.”
Mr O’Halloran said the incident would result in a review of security at the gallery.
“It is not good news for any of us who like going into galleries because we may not be able to get as close to the works as we used to,” he said.
However, Mr O’Halloran remained hopeful that experts at the gallery would be able to restore the painting to its former glory.
“With the quality of restoration that is available nowadays they should be able to get it back into some sort of a form where it will be able to go back on the wall,” he said.