As j26 suggested, the Statute of Limitations for the Revenue is 30 years. Normally a judgment mortgage falls away after 12 years, but a Revenue judgment mortgage does not fall away for 30 years. Converting the IR£66,000 into euro, and applying the judgment rate of interest, the debt is now approx. €170,000.
The Revenue are unlikely to sit on their hands and wait for the 30 years to expire. They have a very effective diary system to keep track of judgment mortgages. They have another 5 years to act, and possibly bring a bankruptcy petition against your Dad.
If your Dad passed away before the 30 year time limit, the debt would have to be paid from his estate etc.
Jim Stafford