。
She found country life boring.
Literal
She [topic-は] country-life [subject-が] boring [quotative-だと] thought.
田舎暮らし ('countryside life') is a compound noun pairing 田舎 (countryside/rural area) with 暮らし (living/life). 退屈だ is a na-adjective meaning 'boring/tedious.' The structure ~が退屈だと思った = 'thought that X was boring' is a straightforward opinion report.