[1] Millerick played in all grades of hurling with Midleton CBS Secondary School before progressing onto the college's senior team.
[2] On 13 December 2017, Millerick won a second successive Dr. O'Callaghan Cup title following Midleton CBS's 1-20 to 0-13 defeat of Gaelcholáiste Mhuire AG in the final.
O'Neill's faced Kilworth in the Cork Premier Intermediate Championship final and collected a winners' medal following the 3-23 to 1-20 victory.
[6] Millerick's performances at club level lead to him being selected for the Imokilly divisional team for the 2017 Cork Senior Championship.
On 22 October, he scored a point from midfield when Imokilly defeated Blackrock by 3-13 to 0-18 to win the Cork Senior Championship final for the first time since 1998.
Lining out at left corner-back, he ended the game with a third successive winners' medal after the 2-17 to 1-16 defeat of Glen Rovers.
[16] He was selected at right wing-back when Cork faced Tipperary for a second time in the All-Ireland final on 24 August 2019, however, he ended the game on the losing side after a 5-17 to 1-18 defeat.