Franck has already played for the French 'B' team, but FIFA rules state that players can change countries as long as they have not played for the full team.
However, further investigations into Franck's Irish ancestry revealed that it was infact his great-grandfather that was Irish and not his grandfather. So, under FAI rules he does not qualify to play for the Republic, by one generation.
Never mind Franck, you'll just have to wait for the call from France.