I realize this question was posted a while ago. Perhaps there were not robust online resources then but there are now many more online Irish / Gaeilge resources such as:
Focloir.ie Comprehensive English-Irish dictionary and pronunciation
Abair.ie Pronunciation tool, input Irish words to hear. You can also speak into this and say Irish words you may have heard to have it try to decipher for you.
Teanglann.ie This actually gets into etymology and grammar and has numerous Irish dictionaries embedded
All three of these have embedded pronunciation audio files in all three Irish dialects (Connacht, Munster, Ulster). Look for the play symbol and C, M, U to click to hear the pronunciation in different dialects.
(Connacht dialect-Western Ireland, Munster dialect-Southern Ireland, Ulster dialect-Northern Ireland. There isn't a dialect for the Eastern portion of Ireland. That is not to say that Irish is not spoken there now; it is. There is just not a dialect due to that area (Dublin/The Pale) being occupied for 800 years - the language did not evolve a distinct dialect in that region).
Back to the original posted question - the difficulty of learning parts of Irish grammar and spelling - especially the eclipsis and lenition - is next to impossible on Duolingo. There are a few YouTube resources now. One lesson that is excellent in this area is Sounds and Spelling of Irish / Fuaimniú & Litriú na Gaeilge
Good luck! Learning Irish is fascinating as it is one of the oldest living languages (along with Basque) in Europe. It is also one of the rarer VSO (verb-subject-object) languages, so interesting for that reason alone.