The tax refund is relative to the amount invested. Say for example, you earned and paid tax on €100,000 of PAYE income in 2011 and were made redundant in early 2012. If you invested €100,000 in a qualifying company, in 2012, that €100,000 would be relieved against your 2011 income. This means that your taxable 2011 income would be nil, and you'd get back all the tax you paid in 2011.
If you invested only €50,000 you're 2011 taxable income would be reduced by that amount and you'd get a refund relative to the €50,000.
You can spread the relief back over the preceding 6 tax years.