The interest quoted is the Annual Equivalent Rate (AER).
If you deposit 10k for 6 months at 4.95% AER then the calculation is simple.
4.95% of 10k is €495. Therefore your term deposit will earn half of that: €247.50.
This interest is subject to DIRT, unless you're over 65 years old. It gets a bit more complex now because the DIRT rate is changing from January 1st 2009 to 23%, up from 20%.
As an example, if you opened the account at the start of November, then two months worth of interest would be subject to 20% tax and the remainder subject to 23%.
1/3 x 0.2 x 247.5 = €16.50
2/3 x 0.23 x 247.5 = €37.95
Total DIRT: €54.45
So you'll receive 247.50-54.45 = €193.05 in interest on your 10k for 6 months.