疲れていたが、彼女は働き続けた。

、。

Although she was tired, she kept working.

Literal

Tired [although-が], she [topic-は] work continued.

疲れていたが uses が as a concessive conjunction ('although'), slightly more formal than けど. The three main Japanese concessives — が, けれど(も), けど — form a formality gradient with が at the top.