... honestly, I'm biased. My wife and I ran a Spanish language school in Guatemala for 30 years. COVID shut it down.

Irish can't be learned academically, imho. You must take the Water Cure: drink 500 liters of water in the Ghaeltacht.

https://www.mezzoguild.com/learn-irish-in-ireland/

It doesn't work for everyone, but I've enjoyed using #anki in the past. It seems #AnkiWeb has some user-contributed #Gaelic lessons: https://ankiweb.net/shared/info/1357645930

On Android, I use #AnkiDroid from #FDroid

Dualingo is pretty unique in it's approach to Scots Gaelic with it's own weird strengths and weaknesses. I find it really useful alongside other things (eg actual classes, or learngaelic.net) rather than a complete learning thing. Like you, I'd rather not use it now it's gone AI... ☹️

Can you elaborate on the shady things? I'm using Duo to learn Irish as well, and if the platform is doing anything questionable, I'd like to know.

Oh, and while this is not an app, I'm looking into it to supplement my learning on Duo.

https://letslearnirish.com

Open Culture lists a couple of alternatives, under "Irish" https://www.openculture.com/freelanguagelessons

I improved my Hindi a bunch with people I met on Conversation Exchange. Basically you connect with people who want to learn a language you know and who know a language you want to learn and practice.

https://www.conversationexchange.com/

are you looking for Scots Gaelic or Irish Gaelic? Either way I don't have an answer but if it is Irish you are going for I am interested in responses.

Shout-out for libraries. Will vary greatly by location of course but some have good language-learning resources to borrow.