All this gender neutral stuff is wearing out my poor brain cells. A banaltra is a female foster father, so an altra is a non-specific memale foster father and a banaltra fir is a female foster father who's done a 180 and become a man again....
I think the British came up with the best, though, when they threw etymology to the winds and came up with midhusband. Move over, Arnold Schwarzenegger.
banaltra is/was like "waitress" or "actress", a specifically gendered term created by adding something to the name of a role that was grammatically masculine. ("created" is probably too strong a word). What is interesting about banaltra is that it was/is grammatically feminine, even though altra is grammatically masculine. Many other -ess words created with the ban- prefix retain the gender of the second part, as is normal for compound words, though another ban- word that crops up in Duolingo, bantiarna, also breaks this rule (bantiarna is feminine, tiarna is masculine). Banríon also appears in one or two exercises, though it seems to be a bit of a tautology as ríon already means "queen" (and is a feminine word).
Here's a few of the grammatically masculine ban- words from the FGB:
ban-aisteoir, m. (gs. -ora, pl. ~í). Actress
banambasadóir, m. (gs. -óra, pl. ~í). Ambassadress
banchigire, m. (gs. ~, pl. -rí). Female inspector
banchliamhain, m. (gs. ~, pl. ~eacha). Daughter-in-law
bandochtúir, m. (gs. -úra, pl. ~í). Woman doctor
baneitleoir, m. (gs. -ora, pl. ~í). Airwoman
banfhreastalaí, m. (gs. ~, pl. -aithe). Female attendant; waitress
banghaiscíoch, m. (gs. & npl. -ígh, gpl. ~). Amazon; heroine
banlaoch, m. (gs. -oich, pl. ~ra). Female warrior; heroine
banmhaor, m. (gs. & npl. -oir, gpl. ~). Stewardess
Some terms, like banaltra and bangharda have been officially deprecated, because they have governing bodies like An Bord Altranais and An Garda Síochána that can make that call. Other terms, like banfhreastalaí, banchigire or banmhaor are probably falling into disuse.