Ah, so it should be. (Fixed above.)

Here it is at Teanglann:

https://www.teanglann.ie/en/fuaim/sionnach

So is the stress on aigh in an tSionnaigh as well? - on tyun-IG?

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WARNING: Intermediate speaker - await further opinions, corrections and adjustments before acting on my advice.
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

It's TYUN-ig

Here's the rule for ach in Munster, in case you are curious:
When the second syllable contains ach and there is no long syllable in the word, the second syllable bears the stress: coileACH, beannACHT, casACHtach

It seems kinda hard to follow at first, but it's actually pretty easy once you get the hang of it.

Thanks for the clarification, a WFM.

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WARNING: Intermediate speaker - await further opinions, corrections and adjustments before acting on my advice.
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), agus Gaeilge an Chaighdeáin Oifigiúil (CO).