I am reading Modern Irish A Comprehensive Grammar by Nancy Stenson. On page 13 she says:

"When one word ends in a short vowel and the next starts with one, only one vowel is usually pronounced. The vowel omitted is usually the unstressed one, most often the last vowel of the first word. If both are unstressed, then, since both are pronounced alike, it is impossible to tell which is lost, but only one vowel is normally heard. Thus,
duine aisteach is pronounced duin’ aisteach
cóta Éibhlín is pronounced cót’ Éibhlín
duine atá is pronounced duine ’tá (or duin’ atá)"

But when I go onto https://www.abair.tcd.ie/ga/synthesis and enter the above phrases both vowels appear, to my untutored ear, to be pronounced. Is this a limitation of the Abair engine, has Stenson got it wrong or am I missing something?

That is a mistake in the Abair synthesis, but you should know that nearly all learners of Irish in Ireland are taught this heavily non-native pronunciation and are insistent that it is right. So anyone you meet who is not from the Gaeltacht would have this duine atá pronunciation and would immediately fly into a rage if you said that is not the correct pronunciation. I kid you not. Galltacht speakers are on a hair trigger over Gaeltacht Irish and are as mad as hell that anyone would suggest the native speakers have better Irish. It has to be witnessed to be believed... No other country does this. I probably would not have studied Irish at all had I know from the beginning about the attitude of the Irish to the Irish language.

Thank you for your response. That seems to be a very basic mistake by the Abair synthesis, which, is after, all supposed to be representing the Irish as spoken in the Gaeltachtaí. I wonder how many other mistakes like that there are in the Abair programming?

So I am clear, do I understand you to be saying that Stenson is correct that the 'correct' pronunciation if you were speaking to a native speaker from Connacht or Munster would be 'duine ’tá' or 'duin’ atá'?

Yes. Nancy Stenson is a very reputable Irish teacher and everything she writes in her book will be worth learning. It is duine 'tá. Or duine 'thá.

Thank you so much. I only have a PDF copy but the book looks a lot clearer, easier to navigate and more complete than my other grammar guides. It is very expensive but I might buy it anyway.