My grandmother's ashes are in an urn placed at a temple.

Literal

Grandmother [possessive-の] ashes [topic-は] temple [at-に] placed urn [in-に] are-contained.

遺灰 (ashes of the deceased) and 骨壷 (urn for ashes) are specific funeral/memorial vocabulary. 寺院に置かれた uses the passive as a relative clause modifying 骨壷: 'an urn that was placed at a temple.' 入っている marks the current state: 'are inside.'