St Aelhaiarn (also spelled Aelhaearn) was a 6th century saint, and as with many of the early Welsh saints, he was a member of the aristocracy, the son of one of the Welsh princes.
Some rebuilding works were in response to damage, including after a fire which affected much of Welshpool in 1665. Earlier that century the church organ was wrecked by Puritans. The current organ was built by the great Victorian organ builder Henry 'Father' Willis.
In the 18th century there were complaints that 'the very common sort of people' went to the gallery at the east end of the church on the pretext of singing psalms but caused commotions there, with some people spitting on the heads of worshippers below!
In the early 15th century the priest here was Adam of Usk. His chronicles are an important historical source, providing information about Owain Glyndŵr’s rebellion, among other things. Another literary figure associated with the church is William Morgan, who completed the first Welsh translation of the Bible in 1588. He was vicar here from 1575 to 1578. There’s a memorial to him in the church.
The oldest object inside is a 1597 memorial to Sir Edward Herbert, a nephew of King Henry VIII. He was the first of the Herberts to live at Powis Castle. A later member of the family, Edward James Herbert (1818-1891), is commemorated by a chest tomb with an alabaster effigy depicting him in academic costume. He was high steward of Cambridge University and Bangor University’s first president. Another memorial honours members of the Montgomeryshire Yemonary who died in the First World War. A brass plaque commemorates Captain Andrew Gordon Reed, who was killed in action in 1915 at Gallipoli, during the Allies futile attempt to invade Turkey.