As you are probably aware legislation has been enacted to make registration of any private sewage disposal system mandatory.
Rather like the household charge many people are avoiding registration as they believe it is the thin end of a large wedge. This is undoubtedly the case but delaying registration will only cost you more money and make your likelihood of an actual inspection more imminent as it suggests you have something to hide.
Register and pay before 28th September and the fee is reduced to €5, they know from planning records and census data who is eligible so you may as well save yourself €45 by registering now.
The EPA website has this to say about Domestic Wastewater Treatment Systems Registration;
The Water Services (Amendment) Act 2012 requires water services authorities to maintain a register of domestic waste water treatment systems in their functional areas. The Local Government Management Agency (LGMA) has developed a central on-line registration system and will manage the system on a shared-service basis for the water services authorities.
Under the Act, owners of domestic waste water treatment systems are required to ensure that their systems are on the register. Your registration will contribute to the protection of Ireland’s ground and surface waters.
All septic tanks and other types of treatment and disposal systems for domestic waste water should be registered not later than 1st February 2013.