[4] She was previously capped for England, the country of her birth, at youth level and received her first senior call-up in August 2019 but never appeared before debuting for Ireland in February 2023.

[7] Mannion began playing football at the age of six for Celtic Reds under the management of Mark Fogarty, captaining the side to a Warwickshire County League title.

[8] Her father would also take her to St Andrew's to watch the Birmingham City men's team play, citing Robbie Savage and Roy Keane as childhood inspirations.

Mannion made 21 appearances in all competitions as Aston Villa won the 2012–13 FA Women's Premier League Cup, beating Leeds United on penalties in the final.

[12] She made her Birmingham debut on 16 October 2013 against Finnish side PK-35 Vantaa in the first leg of the 2013–14 UEFA Women's Champions League round of 32.

[13] Forging a partnership with Kerys Harrop, Mannion played every minute of the 2014 and 2015 FA WSL seasons, with her performances leading her to be named to the 2015 PFA Women's Young Player of the Year shortlist, but lost out to Leah Williamson.

Held at Wembley Stadium, Mannion started the 2017 FA Women's Cup Final as Birmingham lost 4–1 to Manchester City.

[22] She scored her first goal for the club on 12 September 2019, a penalty in a 7–1 Champions League round of 32 first leg victory against Swiss side Lugano.

[24] The injury kept her out for 16 months, finally making a return to play as a 72nd-minute substitute in a 4–0 win over her former side Birmingham City on 28 February 2021, 498 days since her last appearance.

[40] In February 2023, Mannion was named to the Republic of Ireland squad by Vera Pauw for a friendly against China having opted to switch her international eligibility.