Guinan played hurling and Gaelic football as a student at Coláiste Choilm in Tullamore.
[3] After progressing through the juvenile and underage ranks with the Kilcormac–Killoughey club, Guinan made his senior team debut in 2023.
He ended the season with an Offaly SBHC medal after 0–12 to 0–08 defeat of Clodiagh Gaels in the final.
[4] Guinan added an Offaly SHC medal to his collection in 2024, after lining out in the two-point defeat of Ballinamere.
[6] Guinan immediately progressed to the under-20 team in 2023 and was at left wing-back when Offaly lost the 2023 All-Ireland under-20 final to Cork.