Carry out his functions as a garda and asked for reasons why he should not be dismissed.
It is understood that Mr Cooper -- who lives in Clondalkin -- did not reply to this letter and he learned of his dismissal on Thursday last.
A file on the cocaine incident has been sent to the DPP who will make a decision on whether Mr Cooper will be charged in relation to the drugs bust.
In the aftermath of his arrest, Mr Cooper received two months in-house medical care at a residential treatment centre for drug addiction. The former officer has been receiving treatment after his arrest as senior gardai continue to prepare a criminal case against him.
The garda -- whose father Robert was a highly respected detective sergeant in the Ronanstown Garda Station drugs unit before he retired -- was arrested following a sting operation in which he was discovered in possession of 10 grammes of cocaine with a street value of around €800.
A surveillance operation had been mounted for weeks before the arrest in Dublin city centre after Mr Cooper was seen in the company of criminals and senior officers became suspicious of his behaviour.
When the officer met a known drug dealer on Ormond Quay on January 31 -- close to an early house pub where the garda and some of his colleagues were drinking in -- they were both arrested as soon as detectives saw that a drugs transaction had taken place.
After being arrested, the officer was questioned for 24 hours under Section 2 of the Drug Trafficking Act at Kevin Street Garda Station before being released without charge.
Before the garda swoop on the quays, the garda had attended a belated Christmas party at his station -- which had been originally postponed because of the bad weather over Christmas.
The officer had enjoyed a turkey and ham dinner at his station in G District, which covers Crumlin and Sundrive Road.
Afterwards, the garda and two of his colleagues -- one male and one female -- went on a heavy drinking session which ended up at an early house pub close to the city centre.
The investigation into the sacked garda's criminal activities is being led by Det Supt John McMahon of the National Bureau of Criminal Investigation. Separately, the Herald has learnt that Mr Cooper is also under investigation after a drugs bust at the Electric Picnic music festival in Stradbally, Co Laois, in September 2009 .
Mr Cooper was detained after a quantity of LSD 'tabs' were found. The incident is still under "active investigation" and there is a possibility that he may be charged. Meanwhile, a source close to Mr Cooper who was a talented hockey player explained: "He is doing the best he can at the moment.
"He is still getting treatment for his addiction problems and he has co-operated with the investigation in any way he can. Some of the things that have been written about him are rubbish -- there is a feeling that the fella is being hung out to dry.
"He was never involved with gangs or anything like that."