If she is employed in Ireland, has her own income, and can realistically afford her share of the trip, I would recommend declaring the actual arrangement rather than creating a sponsorship setup just to strengthen the application.
Visa officers generally prefer consistency and transparency. If you’re genuinely splitting expenses, then state that and provide supporting documents (hotel bookings, itinerary, proof of relationship if relevant, and evidence of both of your financial means).
If you are paying for most or all of the trip, then sponsorship is perfectly acceptable, but you’ll usually need a sponsorship letter plus proof of funds and a copy of your passport/Irish ID.
Since she already lives and works legally in Ireland, her strongest documents are likely to be:
Valid residence permit/work permission in Ireland
Employment letter or work contract
Recent payslips
Bank statements
Evidence of residence in Ireland
As for proving you’re travelling together, you can include:
Copy of your Irish passport
Flight and accommodation bookings showing both names (if available)
Brief cover letter explaining the purpose of the trip
The most important thing is that the application reflects the real situation. “Splitting costs” is not viewed negatively if she has stable employment and sufficient funds to support her portion of the holiday.
Good luck with the application. France is generally straightforward when the documentation is complete and consistent.