Only people in PAYE employment can avail of the PAYE credit, so self-employed people (even if they pay tax under the PAYE system) cannot avail of this.
The married tax credit is a seperate issue. You can allocate it between you any way you like. So if you keep all the credits, yes, your wife will indeed pay tax on the first euro she earns. She will pay tax at 20% on the first 25k, unless your taxable income is less than 43k, in which case she can add the difference between your salary and 43 k to her 20% band (in other words, if you were earning 40k, she could have up to 28k taxed at 20%.)
She'll also pay PRSI and health levies, of course, at 5% as a self-employed person