These type of septic tanks are often 25 years old or more, according to the Meath-based expert.
Another common problem is that septic tanks were installed in areas where the soil was unsuitable for percolation.
"You'll often see this with a tank 10-12 years old," said Mr O'Reilly.
"We will get a call from someone who says their septic tank is not working. When we go out, we find that the tank is structurally sound, but the percolation area is not suitable."
In these cases, the soil will be tightly packed and have very little gravelly content, resulting in poor soakage.
"The tank will have percolated fine for a number of years, but then it fails," he explained.
"In a case like that, the upgrade would involve adding a second tank to take effluent from the first tank where the bacteria can further digest the solids. This would increase the efficiency of the system from about 30pc to 97-98pc," he explained.
"County councils pay a lot more attention to the results of the percolation test these days," Mr O'Reilly added.
In the percolation test, if you get a P result of under five, water can move too quickly through the soil to the groundwater underneath. Typically these soils are gravelly or sandy soils.
If the P result is between five and 20, this is positive and means the soil is a suitable percolation site.
Results between 20 and 50 indicate that some remedial work will be required to make the site suitable, while a P result over 50 does not disperse liquid well and another system will be required.
Householders are also often guilty of not paying attention to desludging their septic tanks, according to Mr O'Reilly.
"You'll often hear someone saying they 'got away with only a septic tank' in the site requirement," he explained.
"Yes, a septic tank is definitely the cheapest option, but that does not mean you can forget about it the minute it goes into the ground," he warned.
Large septic tanks capable of holding four cubic metres of water, attached to a household of two adults and two children, need to be desludged every three years on average.
However, smaller tanks and tanks for houses with more people will need to be tackled more often.
"You'll hear of people who have never desludged their tank in 10, 20 or even 30 years, but as the tank fills up with sludge, it will reach the pipe and spill out into the percolation area and stuffs it up."
To check if your tank needs to be desludged in a single-chamber tank, take a piece of 2x1 timber and push it down through the crust until you feel the resistance of the sludge below.
Mark this level on the timber. Next, push the timber right down to the bottom of the tank and mark this level also.
The tanks should be one third sludge and two thirds water. If the sludge component is higher, the tank needs desludging.
"Most septic tanks systems in Ireland are way too full of sludge and this compromises the ability of the bacteria to digest the material," maintained Mr O'Reilly.
Finally, another common problem with septic tanks is the overuse of biological washing powders and chemicals that kill off the bacteria that areneeded to digest the waste in your tank.
"Those chemicals contain enzymes that kill off all bacterial activity, so the active biomass is no longer there to deal with waste.
"You will get a smell from your tank because what you have in it is just raw sewage," he said.