There are a lot of variables here.
Assuming they sell their family home (PPR), which has always been their PPR then they will pay no tax on sale of this property.
Pretty much all income in Ireland is taxable, how much tax you pay depends on circumstances.
Indicatively, for over 70, contributory state pension is approximately €12,500/year and you can earn approximately another €3500 without paying any tax (you will pay a small amount of USC though)
Assuming they were targeting an income of €12,000 after all costs, annual net income from the rental would be just under €10,000.
If they have other sources of income, these calculations will differ - can you give a bit more background?
If the rental property was sold after a period and the value had increased, the increase in value is subject to capital gains tax @ 33%.
Regarding other considerations, bear in mind that rental properties require maintenance, property tax, agents fees, PTRB registration etc etc - there can be years when there is zero profit from a rental following a bad tenant etc.