Just wanted to update and thank everyone for their help on this problem.
My husband was granted his Irish Foreign Birth status on the basis of the Edmond Tobin/Margaret Maher birth certificate. I'm still not 100% sold that this is the right family, but it's the closest we've been able to find and the government asked no questions. So we're safe from Brexit for the mean time.
I'm still trying to trace the family so any new info appreciated.
Thanks again.