Well I wouldn't say "Nothing", you going to get billed sooner or later. It was your responsibility to register your property for the household charge back in 2012 so saying you never got no correspondence EVER won't wash with Revenue. They was enough media coverage about it at the time for you not to have heeded it.
The Household Charge (Due for 2012 ONLY)
The Household Charge (HC) was an annual charge introduced by the Local Government (Household Charge) Act 2011 which was payable by owners of residential property. It was a matter for owners of residential properties to register and pay the HC on or after 1 January 2012.
1. Household Charge Arrears (payable to Revenue ONLY)
From 1 July 2013, any outstanding HC liability (including late penalties) was increased to €200 and is payable to Revenue.
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So you now owe €200 for the household charge which was due in 2012. As you didn't register your property, you didn't receive Revenue's estimate for the property tax which came into force in 2013. As far as I am aware there are no penalties so far for failing to pay the property tax so your best bet is to contact Revenue and come to some sort of a payment agreement to pay the arrears for the amount due over the last three years.
Bear in mind you cannot sell your property until all such charges have been paid.