[4] A qualified schoolteacher and fluent Gaeilgeoir, Cassidy's business ventures include the pub Teach Mhicí and Coláiste Chú Chulainn — a summer residential Gaelscoil for children of primary school age.

Kevin, his brother Stephen and his five sisters were born in Glasgow where they lived until Cassidy was eight years of age.

He returned later in the summer to win a second Donegal Senior Football Championship with Gaoth Dobhair, beating St Eunan's in the 2006 final.

[9] He was then instrumental in Gaoth Dobhair's first ever Ulster Senior Club Football Championship success later in 2018, a run which involved a quarter final defeat of Cargin (a game in which Cassidy scored a goal and three points),[10] a semi-final defeat of eleven-time winners Crossmaglen Rangers (a game in which Cassidy scored a goal and two points),[11] and culminated in the club defeating Scotstown (who had won the competition on four previous occasions), the last two of these games occurring at Healy Park in Omagh.

[3][7] Thus, Cassidy added a provincial club title to his three county championships, and, afterwards, he described it as "the proudest moment" of his life.

[12] Cassidy earned praise both for his performance and, also, his efforts to control his fellow players and management team when tensions threatened to escalate on the pitch.

[3] Afterwards, he reflected on his expulsion from the county team by Jim McGuinness (see below) and admitted that if this had not occurred, and taking into consideration his then age of 37, he would most likely have been a spectator instead of a player on the day.

[8] Then he famously went on the lash when Donegal secured a draw against Dublin in their August Bank Holiday Monday All-Ireland quarter-final.

He made a substitute appearance in the first game of Brian McEniff's last spell as Donegal manager, a league defeat to Galway in Tuam in February 2003.

[18] However, with Fermanagh having knocked Donegal out the Ulster Championship, the team entered the All-Ireland Qualifiers, with their first game coming against Longford on a Saturday evening in Ballybofey.

'[16] A Kevin Cassidyless Donegal overcame Longford, and ultimately reached the All-Ireland semi-final where they sustained a narrow loss to Armagh at Croke Park.

[19] In 2004, Cassidy helped Donegal to reach the Ulster Senior Football Championship Final which they lost, again to Armagh.

[25] Cassidy made his 100th appearance for Donegal in Ballyshannon against Laois in the 2010 National Football League; in the ninth minute he began a move that led to a Conall Dunne goal.

[27] Cassidy went on to help Donegal to their first provincial title in 19 years with the defeat of Derry in the 2011 Ulster Senior Football Championship Final.

[30] This thrilling end to an "extra-time epic" is to this day regarded as "the finish to a sporting contest normally only imagined inside the heads of Hollywood scriptwriters".

[31] In November 2011, manager Jim McGuinness dropped Cassidy from the Donegal team after he contributed to a book (This Is Our Year).

In what went down as a "surreal moment for the viewer", Mícheál Ó Domhnaill famously interviewed McGuinness following a live 2012 league game on TG4 while Cassidy, in the role of television analyst, stood beside him with his head bowed.

Cassidy described the team's homecoming, with Gaoth Dobhair's GAA pitch located just across from his house: I was sitting eating Weetabix about half eleven at night.

Cassidy responded by referring to his Glasgow birth and his soccer heritage: I really loved playing for Donegal and I was really proud to do it… [But] it wasn't a childhood dream to win Sam Maguire, if I am being honest… I was a soccer player and then the first thing that turned me was [my brother] Stephen playing for Gaoth Dobhair and they won a Ghaeltacht Championship.

That was my first introduction to it and then I met [my wife] Sarah [Gallagher, whose father Willie played for Donegal]… obviously her history and family was steeped in it.

Tonight he reveals in Laochra Gael that his 2011 Ulster medal, the only inter-county provincial title he'd win, went with my father to his grave".

[48] In November 2020, Cassidy returned as Gaoth Dobhair manager ahead of the 2021 season (succeeding Mervyn O'Donnell), jointly with Joe Duffy, and with Maxi Curran as team trainer.

In July 2012, Cassidy was found guilty of affray and sentenced to community service, the judge noting that none of the men involved had appeared in court since that time and none had had any previous offences like it.

[6][50][51] With an associate, Hugh McGinley, Cassidy operates Coláiste Chú Chulainn in Gaoth Dobhair.

[7] Squad as per Gaoth Dobhair v Scotstown, 2018 Ulster Senior Club Football Championship final, 2 December 2018