So her Irish accent lasted exactly one episode?

it will recur a few times, mostly in "back story" (memories) that are in season 7.
her "read" for the part was done in an irish accent and was very good in that. (this is the audition for both Jeffrey Donovan and Gabrielle Anwar, she is about 5 minutes in):

https://www.youtube.com/watch?v=yzH6IrOHB5g

I don't know if you think it was just "dropped" without explanation or just making a general statement, but it was explained within the first few episodes that it was a conscious decision on Fiona's part (the character, not the actress) to start using an American accent since she's living in Miami now. It was written as part of the show, not just "dropped".

I don't imagine the "powers that be" thought fans would survive the entire series with that Irish accent (no offense to Irish peeps). It just would have been too much effort for good ole American boys and gals to try to suss out what a major character is trying to say in every episode in that sing songy voice, and they probably feared people would tune out after a while. And Americans aren't big on using closed captioning, so the only way was to write it out.

Plus I doubt Anwar wanted to keep putting on an accent day in and day out. I like her American accent, it was a welcome change in my opinion.

It is just like the Michael Westin character, he is American, but when he went under cover in Ireland, he used an Irish accent, back home he uses his American accent. Fiona is in Miami, why stand out with an Irish accent, she does have enemies, might as well blend in.

She said in the beginning of episode 2 "I can't walk around sounding like a friggin leprechaun can I"