[3] Tadhg grew up playing Gaelic football and was recognised as a highly talented underage player in Kerry GAA.

[6][7] After his debut in 2001 (following elevation from the rookie list), he became a permanent fixture in the Sydney line-up as a dashing rebound defender, wearing the number 17 guernsey.

Kennelly earned an AFL Rising Star nomination in Round 19 of 2002, consolidating his reputation as an upcoming young talent.

The hype in Australia surrounding Kennelly's appearance in the AFL Grand Final was so much that Network Ten sent out its usual game-day boundary rider, Andrew Maher, over to County Kerry to visit Tadhg's family for the week and watch the Grand Final with them.

[10] In 2007 and 2008, after several seasons missing only a handful of games, Kennelly suffered a series of serious leg and shoulder injuries which became major setbacks to his AFL career.

He was selected in the Dream Team for the AFL Hall of Fame Tribute Match,[11] but he later withdrew due to injury.

During the rest of 2008, Kennelly's shoulder continually dislocated during games, and his unorthodox methods of popping it back in was the subject of much media interest.

[12] On several occasions during his AFL career, Kennelly had expressed a desire to return home to Ireland and leave the AFL, as he wanted to emulate the success of his father and win an All-Ireland Senior Football Championship medal with his native county's team, Kerry.

[19][20] He went on to win an Irish NFL medal when Kerry defeated Derry in the final at Croke Park.

[23] The book was controversial; a paragraph claimed that he set out to make his mark at the start of the All-Ireland SFC Final by deliberately colliding with Nicholas Murphy.

[25] On 12 November 2009, Kennelly signed a two-year deal with the Sydney Swans, marking his return to the AFL.

[29] In 2021, he founded a fitness class and discussion group aimed at offering peer support called "When No-One's Watching".

[30] In October 2022 Kennelly was appointed Academy coach of the Swans cross town rivals Greater Western Sydney Giants.