There is no tax advantage in spreading it out over periods within a tax year.
You should do up a short document with a few examples to illustrate. Case 1 - €3,000 per month. Case 2 - 2,000;2,000 and €4,000. Case 3 - €4,000, 2,000 and 2,000. They will see that the cumulative tax for the three quarters is the same.
If you are due to pay bonuses in January, it often saves tax by paying them in December. This is particular good for people who have just returned from abroad and have not used their tax credits. Bonuses paid in 07 may be taxed at 0%, whereas bonuses paid in 08 may be taxed at 41%.
PRSI is a different matter
Employees pay a higher rate if they exceed certain monthly thresholds. They can claim a refund at the end of the tax year, but most don't. It is worth spreading out bonuses for this reason alone.
These are the Key Figures which someone doing payroll should keep in front of them. Read the thread about Explaining Prsi, for more information
€1525 - no employees prsi
€1543 - employers' prsi is 8.5% instead of 10.75%
€2166 - employees pay 4%
Brendan