Fuaimeanna is better in general as Teanglann sometimes has weaker speakers for some words. But both use fully recorded audio, not AI.

It's a very interesting area of research but it's not without its downsides. As for whether the research team at TCD think the AI model is currently more accurate than the earlier speech synthesis engines, I think probably not. I imagine the benefit of it to them is that they don't have to manually tweak the AI model as you would with a rule-based model (which I expect is how the earlier engines worked).

The thing about AI speach synthesis is that it's only as good as the data which is available to train it. These things need a lot of data (recordings of native speakers mapped to written text), and it needs to be good quality data. Unfortunately, good quality recordings can be difficult to get for Irish. Native speakers have better things to do than sit around making and transcribing recordings of themselves, and a lot of people are uncomfortable with recordings of them being used to train AI anyhow (just look at the trouble Apple have with Siri, and they have millions of users). I suspect the AI model the TCD team are using is pretty good, but where you get weird examples of words it doens't pronounce well, it's probably because they don't have sufficient pronunciation data for that character sequence to train the model on.

The upside is that, as more speech data becomes available over time, improving the model should be as easy as running a script to retrain on the new data. The downside is that it's impossible to know when enough data may become available for training this kind of model to the point that we can safely say it's better than the synthesis engines they originally used. I think moving to an AI model was probably the right choice. It's certainly the forward-thinking approach to take. But it may have been just a little premature.