She was speaking Gaelic.
Killing Can Be a Righteous Act.

At first I thought the foreign tongue was because Agnes had been possessed by the Butcher. But since the Butcher killed her, and she was not possessed, I think it's definitely possible she just learned a few choice phrases while method acting/fangirling the Butcher.
Or, maybe Agnes is possessed by the witch and that's how she learned? I would be down with that. Adds another layer to Bates' performance.

Hah, I know what you meant. And I agree

That racket kid is literally the worst.
Less than forget. But more than begun.

You really just don't get it. Forget about Kathy Bates.
Agnes - the character - learned the Celtic/Gaelic on her own. She was not acting or performing when she uttered those lines. She decided to learn those on her own - BECAUSE SHE IS CRAZY.
Stop overthinking it.

Kathy Bates as Agnes and Kathy Bates as the Butcher was still memorizing Ryan Murphy's screenplay for the episode. It's not like she made those up on the spot and violated Murphy's screenplay.

Kathy Bates had to learn the phrases in the script to play Agnes.
Agnes the character really threw herself into the role of the Butcher, and no doubt learned phrases when she showed up at the house, violating the restraining order.
Gee Woodle, Space Kadoodle!

Learned from phrases? It's not like she ad-libbed the scene. It had to have been in the screenplay.

Agnes really threw herself into her research for this role, no doubt she learned some phrases!
Gee Woodle, Space Kadoodle!

Ok that makes sense! I had thought it sounded like Celtic or something similar but wasn't sure how Agnes would even know how to speak a language like that

Yes, I often put on closed captions if there is background noise, and Agnes was in some portions speaking a foreign language.
Like a PP said, sounded Celtic.
Gee Woodle, Space Kadoodle!

I believe she was speaking in a Celtic language
Avada Kedavra muthaf

Her "Ye Olde English" accent is beyond terrible.