You need specific tax advise on this, preferably from an experienced and reliable tax consultant. There are strict time limits within which Revenue can raise assessments for unpaid taxes on the estate of a deceased person, however I suspect that the fact that your father's estate doesn't seem to have been finalised as yet will make these limits moot in this case.
I'd also wonder about whether the €100k CGT liability is real or not. He'd need to have sold the business for a phenomenal sum 5 years ago to generate such a big liability on the resulting gain and if he was aged over 55 years, and had owned and run the business for more than 10 years, and met a few other conditions, he would presumably have been entitled to retirement relief which could have wiped out the full liability. If he had an accountant or tax advisor at the time, you should first check this with them.