In 2000, aged 11, Agnew approached director David Downes about recording a song to raise money for the children of Afghanistan.

With his help, she recorded Angel of Mercy for the album This Holy Christmas Night, which raised over £20,000 for the Afghan Children's Charity Fund in 2001.

She appeared as part of the group Celtic Woman at The Helix in Dublin in 2004, the youngest member at the time.

Agnew has been featured on 14 albums with the group, recording songs in multiple languages and taking part in several world tours.

Agnew was also part of Ethan Bortnick's concert with another former Celtic Thunder member Damian McGinty.

Agnew did a concert performance with her boyfriend Dermot Kiernan and American singer-songwriters Kate Steinway and Marissa Lauren in the LA Songwriting school on 2015.

On 22 January 2020, it was announced that Agnew was returning to Celtic Woman for the US Celebration Tour to fill in for Éabha McMahon, who was taking a leave from the group.