Harry Clarke was one of the great masters of 20th-century stained glass. He trained in his father’s studio and at the Metropolitan Art School in Dublin, which gave him an intimate understanding of the nature of glass. He soon learned to employ sophisticated techniques to create decorative effects. He toured Europe to study medieval glass, and was especially inspired by the 12th-century glass in the French cathedral of Chartres. Many of his windows, such as this one, are reminiscent of those at Chartres in the depth and richness of their colour schemes.

Clarke was especially skilful in exploiting the qualities of the new slab glass, which E.S. Prior had invented in 1889. The irregularities in the thickness of this coloured glass mimicked that used in medieval stained glass. Clarke enhanced the qualities of this richly coloured glass by acid-etching, the application of stains and fine delicate painting.

He created the design for the Apparition of the Sacred Heart in 1918. This was in preparation for a window for Phibsborough Church in Dublin, commissioned by the Arch-Confraternity of the Sacred Heart. The window was completed and installed in 1919. Subsequently, Clarke and his studio reused the original design to make other Sacred Heart windows.

The devotion to the Sacred Heart of Jesus Christ dates back to medieval times, but it did not become a feast of the Catholic church until 1670. The spread of this devotional feast was largely due to the visions of St Margaret Mary Alacoque (1647–90), who communicated that Christ wanted to reveal his ardent love for mankind. He wished to manifest his heart with its abundance of love and mercy. Margaret Mary was canonised in 1920.