"Cad a tharla?"
Only in its past tense. It’s among a handful of verbs that are slightly irregular and aren’t listed in the usual set of eleven irregular verbs.
This is pure speculation on my part, but because Irish doesn't have Text to Speech audio, and therefore only has sound on a limited number of exercises, somebody must have made a list of exercises that were suitable for "Type what you hear" exercises. This wouldn't be necessary for most languages, which use TTS, because all sentences can be spoken, so therefore any sentence can be used in "Type what you hear" exercises. When the audio was being re-recorded, a different list of sentences was used to create the recordings - some old sentences that had audio don't have new audio, and some old sentences that didn't have audio have audio now. But whatever "hack" was put in place to limit the "Type what you hear" exercises to just the list of sentences that have audio wasn't updated with the list of new recordings.
Again, I'm not saying that this is definitely what has happened, it's just the simplest explanation. But if I'm right, either AlexinIreland will have to go manually fix each sentence as it is reported, or else one of the Duolingo staff (the course contributors like Alex are not Duolingo staff, and don't have access to the actual Duolingo infrastructure) will have to go back and rebuild whatever list is being used to get around the fact that Duolingo can't use every sentence for "Type what you hear" exercises in Irish. The second approach would be the fastest solution, but getting Duolingo staff attention doesn't seem to be easy!
Can this sentence be translated with "What has happened?" or is this strictly past tense?
Depending upon context, the Irish past could be translated with the English present perfect. The Irish present perfect has a different structure, and it isn’t used in every situation where English would use the present perfect.