I don't know the specifics of Irish law, but the free movement directive (2004/38/EC) establishes with articles 10 and 23 that the right to take up employment should be independent of any document:
Irrespective of nationality, the family members of a Union citizen who have the right of residence or the right of permanent residence in a Member State shall be entitled to take up employment or self-employment there.
Irish law should therefore provide that you have the right to work in Ireland as soon as you arrive there with your spouse (or join your spouse there).
Issue of residence cards
1. The right of residence of family members of a Union citizen who are not nationals of a Member State shall be evidenced by the issuing of a document called ‘Residence card of a family member of a Union citizen’ no later than six months from the date on which they submit the application. A certificate of application for the residence card shall be issued immediately.
The card is evidence of the right to reside (and, under Art. 23, to work). The card is not the source of that right. (This is why it's called a "card" rather than a "permit.") Rather, the source of the right is found in Chapter III, which is perhaps to lengthy to quote here, but it's worth noting that the right of residence for up to three months (Art. 6) exists even without any obligation to obtain a residence card.
From a practical point of view, it may be difficult to find formal employment without the card for administrative reasons, but if you were to do work, for example informally, that should be lawful. This of course implies that you are legally permitted to look for work before you receive the card.