I'm afraid not.
Using Brendan's example, the additional tax payable is €6,120 leaving an after tax gain of €5,880.
The effective rate on the gross income is 33.74%.
This does not mean that the €12,000 rental income should be taxed at 33.74%, but rather that the tax rate averaged on €62,000 is 33.74%.
Brendan's example was easy to work out as the lower rates of income tax and USC were used up by the salary.
In other cases you might have to take account of unused portions of lower rates.
That said, your point regarding incomes subject to progressive rates and incomes subject to flat rates is legitimate.