(a) The PAYE credit is not allowed as mentioned in spouse-spouse employments.
(b) If the spouse(wife in your case) actually works in the business for pay then that is perfectly aceptable and is an allowable business expense. She can then earn up to the €26,400 at the lower rate. In this case the employer spouse registers as an employer and gets a proper certificate of tax credits for the employed spouse. Pay is then made to the employed spouse subject to the tax credits allocated.
(c) Spouse to spouse employments are not insurable under the Social Welfare Acts. Accordingly no PRSI is payable. The employment would be recorded at class M. However If the threshold for the Health Contribution Levy is reached then that 2% is payable and the employment is recorded under class K1.
Depending on the business you often find that the other spouse is heavily involved in the business but not on any official footing. e.g. the farmers wife who does the early milking 6 out of 7 days of the week , the builder whose wife takes calls, does the books, generally keeps things running in the office and so on. In such cases why should they not be paid for their work.