Donegal had struggled to beat Carlow and Clare in the previous rounds, and Derry's impressive dismissal of Monaghan last weekend made them worthy pre-match favourites.

But Donegal's first-half showing and an injury to Derry's key forward Paddy Bradley, who was taken off after 15 minutes of the second half, made it anyone's game.

Colm McFadden netted after 18 minutes to help the hosts towards a 1-05 to 0-05 half-time lead.

Losing an influential player such as Bradley meant Derry had to dig deep to stay in touch and they did just that, levelling the game in dramatic fashion with an injury-time point from Chrissy McKaigue.

McKaigue's effort set up two periods of extra-time, with the sides locked together at 1-11 to 0-14.

Donegal just had enough in those added periods to get over the finishing line, aided by Cassidy's well-taken goal.

Derry launched a spirited comeback and missed out on sending the game to a replay when Enda Lynn failed to hit the target in the dying seconds.

Donegal made two changes to their selected team, with Michael Boyle getting the nod in goal and Brendan Boyle named in midfield.

Meanwhile, Chrissy McKaigue came in as the visitors' one change to the side that beat Monaghan.

Donegal hit the ground running, pumping the ball in towards Michael Murphy who notched their opening point with a lovely, first-time shot.

The Tir Chonaill men were certainly dictating play but they could not transfer that dominance onto the scoreboard, and James Kielt levelled for Derry after 15 minutes.

Mark Lynch doubled Derry's tally, three minutes later, but then McFadden used all his experience to slot the ball past Barry Gillis and push Donegal into a 1-01 to 0-02 lead.

McFadden ended the first half with 1-02 as the sides headed for the dressing rooms with John Joe Doherty's men holding a three-point buffer.

The other scorers in the latter stages of the half were Lynch, Kielt, Barry Monaghan, Paddy Bradley and Brian Roper.

Donegal took over in the early stages of the second half, their dominance coming after Bradley had registered an early free.

Murphy replied with a pointed free and a score from play and Kevin Cassidy made it a five-point game. In between, Conal Dunne was inches away from claiming Donegal's second goal.

Despite losing Bradley to injury, Derry clawed it back to 1-10 to 0-10 thanks to successful efforts from substitute Gavin McShane, new free-taker Barry McGoldrick (0-02) and Kielt.

The tide was turning slightly and two superb points from Eoin Bradley - one on the run from 40 yards out - put just one between the sides as the hour mark passed.

A free from Murphy gave Donegal some comfort, yet Derry gave themselves a late chance when Lynch launched a brilliant effort through the uprights from far out.

Up stepped McKaigue, in his first year of SFC football, to assume the weight of responsibility and kick the score which sent the game to extra-time.

The momentum was most certainly with Derry at this stage, but although Eoin Bradley kicked them ahead at 0-15 to 1-11, Cassidy quickly replied with a well-taken goal that was to prove so crucial.

Donegal turned around at the end of the first period of extra-time, leading by 2-12 to 0-16. They should really have been further ahead but their shooting let them down, albeit with David Walsh replying to a Gerard O'Kane point.

Derry reduced the arrears to a single point once again, courtesy of a pinpoint Kielt free, but Donegal put a succession of wides behind them when Murphy landed a well-executed score after a brilliant fetch by Rory Kavanagh.

Kielt kicked a free for Derry, which proved to be their final point of their 2009 Championship. Lynn failed to convert a last-gasp chance and Donegal held firm to pick their third successive home win in the qualifiers.