。
She really likes that dress.
Literal
She [topic-は] that dress [subject-が] is-pleased-with.
気に入る ('to like, take a liking to, be pleased with') uses が for the thing liked, parallel to 好き. Literally it's 'enter [one's] spirit' — the thing comes into one's heart. ~ている marks an ongoing state of liking; 気に入った (plain past) marks the moment liking began. 気に入っている is closer to 'has come to like and continues to like'. 気に入る is somewhat warmer and more colloquial than the more abstract 好き; you might 好き a celebrity but 気に入る a particular dress, item, café, or new acquaintance.