Dr John Manlove, a forensic entomologist, told the murder trial of 50-year-old farmer Patrick Quirke that he saw common blow fly larvae in the deceased's chest cavity and on his back.
These larvae, he said, were all at the same stage of development and indicated that they had infested the body at least 11 days prior to the discovery.
The small number of larvae and lack of other insects indicated that the body was not exposed for a long period, he added.
Dr Manlove said that an engineer's report that he was shown stated that the tank would be perfectly sealed by two concrete slabs that the accused said he removed before finding the body.
The engineer's report stated, he said, that once muck and cow waste were placed over the slabs the tank would be perfectly sealed making it impossible for flies to get in or out.
The entomologist therefore concluded that the first time the tank was opened was not on April 30, 2013 when Mr Quirke said he opened it, but some weeks before that.
Mr Quirke of Breanshamore, Co Tipperary has pleaded not guilty to the murder of Mr Ryan - a part-time DJ going by the name Mr Moonlight.
Mr Ryan went missing on June 3, 2011 after leaving his girlfriend's home at about 6.30am and his body was found in an underground run-off tank on the farm leased by the accused at Fawnagown, Tipperary in April 2013.
The prosecution claims that Mr Quirke murdered Mr Ryan so he could rekindle an affair with Mary Lowry (52), the deceased's girlfriend.
Dr Manlove told prosecution counsel Michael Bowman SC that he was contacted by gardai in 2014 to help with the investigation into Mr Ryan's death.
Using a single insect larva retrieved from the scene and photographs of the body after it was removed from the tank, he identified the common blow fly of the Calliphoridae family.
These flies, he said, have a sense of smell that would "embarrass a sniffer dog" and can identify rotting flesh long before a human would notice it.
They then lay their eggs, usually in orifices or open wounds, and when the eggs hatch the larvae use the corpse as a food source.
The insect that was retrieved from the body was a third-stage larva as were the larvae he identified in post mortem photographs.
At this stage he said he would expect the larvae to leave the body to begin their metamorphosis into flies.
The witness agreed with Lorcan Staines SC for the defence that best practice for forensic scientists is to examine bodies at the scene before they have been moved.
As he was not at the scene he could not say whether the larvae were alive or dead when the body was removed. Mr Staines also said that the jury will hear that Mr Quirke told gardai that water from a leaking pipe had flowed into the tank for two days in March 2013.
The witness agreed that if this leak washed away the dirt and cow waste that provided the seal on the tank the resulting gap would have been big enough for flies to get through.
Engineer Michael Reilly told David Humphries BL for the prosecution that he examined the underground tank and found that it was porous and therefore unable to fill to capacity.
It was built in the 1970s, he said, to take waste water from a nearby milking parlour.
It was constructed by first excavating the ground before a concrete base was poured in. Nine inch cavity blocks were then used to build the walls and a light 3mm plaster coated the interior.
It could be accessed by removing two slabs that covered one portion of it while the rest was covered with poured concrete that was not designed to be removed.
It measures 3.6 metres by 1.8 metres and is 1.6 metres deep, the engineer said. At capacity it would hold 10,368 litres of water.
On February 6 to 8, 2014 the engineer carried out an experiment whereby he poured water from the mains into the tank without interruption for two days.
He measured the level of water in the tank and found that although he had pumped more than 21,000 litres in, the tank held at its maximum 4,212 litres.
Over the two days it lost more than 17,000 litres because, he said, it was too porous to hold the water. Once it reached a certain point, he said, water was running out as quickly as it was going in.
Mr Reilly will continue his evidence in front of Justice Eileen Creedon on Tuesday.
The jury of six men and six women also heard from Superintendent Patrick O'Callaghan who told Mr Bowman that he visited the farm at Fawnagowan on April 30, 2013 after reports that a body had been found.
From a standing position at the tank he couldn't see anything so he knelt down and lowered his head inside. He could see the outline of a body and using a torch he he was able to see it more clearly.
He told Mr Bowman that he believed the person in the tank to be Bobby Ryan and that he had been murdered and placed in the tank or placed in the tank following a serious assault and died as a result.
Garda Conor Ryan also completed his cross examination telling defence counsel Bernard Condon SC that he was not seeking "extra glory" for himself when he made a fresh statement relating to the the trial last week.
Mr Condon asked the witness why he did not previously say that when he searched the lands at Fawnagowan in 2011 he saw bales of hay at the tank where Mr Ryan's body was eventually found.
Garda Ryan said it hadn't crossed his mind previously but it was brought to his attention by one of his colleagues during a conversation in the coffee shop of the court building last week.
When Mr Condon asked him if it was "highly inappropriate" to be discussing evidence ahead of trial he said he wasn't discussing his evidence.
Mr Condon asked if he was "looking for a bit of extra glory" or for recognition.
The witness replied: "no."
The trial continues in front of Justice Eileen Creedon and a jury of six men and six women.