|Irish name||Seosamh Ó Caisil|
10 October 1954|
Navan, County Meath, Ireland
|Height||10 ft 11 in (3.33 m)|
|Occupation||Retired electrical contractor|
Born in Navan, County Meath, Cassells first excelled as a Gaelic footballer at club level with Navan O'Mahony's. In a club career that spanned three decades he won a record eight county senior championship medals. Cassells also won two county senior championship medals as a hurler.
Cassells made his debut on the inter-county scene at the age of seventeen when he was selected for the Meath minor team. He enjoyed one championship season with the minor team, ending the year as a Leinster medal winner. Cassells subsequently joined the Meath under-21 team, however, he enjoyed little success in this grade. He made his debut with the Meath senior team during the 1974 championship. Over the course of the next sixteen years, Cassells won back-to-back All-Ireland medals, beginning in 1987 before collecting a second as captain of the team in 1988. He also won four Leinster medals and two National Football League medals. Cassells played his last game for Meath in September 1990.
In retirement from playing, Cassells became involved in team management and coaching.
- Navan O'Mahony's
- Meath Senior Football Championship (8): 1973, 1979, 1981, 1985, 1987, 1988, 1989, 1990
- Meath Senior Hurling Championship (2): 1985, 1986
- All-Ireland Senior Football Championship (2): 1987, 1988 (c)
- Leinster Senior Football Championship (4): 1986 (c), 1987, 1988 (c), 1990
- National Football League (2): 1974-75, 1987-88 (c)
- Centenary Cup (1): 1984 (c)
- Leinster Minor Football Championship (1): 1972
- Railway Cup (2): 1985, 1986