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  MacNiallais' first goal contributed to Donegal's early 1-5 lead to 0-3 lead.
  Fermanagh cut the margin to two and had a chance to lead before the break after Neil McGee's red card but Mark Anthony McGinley saved Sean Quigley's penalty.
  However, MacNiallais's second goal on 44 minutes put Donegal in firm control as Fermanagh's play became ragged.
  MacNiallais was perhaps fortunate to register the second goal after a superb move with his left foot appearing to be in the small square after taking a pass from Martin O'Reilly.
  Donegal's win sets up a provincial semi-final against holders Monaghan on 25 June as the teams involved in the last three Ulster deciders meet again.
  Fermanagh, meanwhile, will face Wexford away in the qualifiers in two weeks.
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  Quigley's penalty miss was the key moment of the contest as Donegal championship debutant McGinley was able to make a comfortable save.
  Donegal were leading 1-6 to 0-7 when the penalty was awarded after McGee had been red carded by Maurice Deegan for landing a retaliatory punch on Ruairi Corrigan after he had been fouled by the Fermanagh man.
  The missed penalty was the final play of the opening half as Donegal remained ahead at the break.
  Despite being a man down, Donegal took control in the second period as MacNiallais palmed to the net in the 44th minute following a sweeping move after a Fermanagh attack had broken down.
  Key Fermanagh defender Che Cullen, who had bossed a struggling Michael Murphy in open play in the first half, was black carded moments later and Donegal added a further three unanswered points to put the outcome beyond doubt.
  Ryan Jones had a goal chance with 10 minutes remaining which would have cut Donegal's lead to only four but his shot skimmed over the bar and with that, went Fermanagh's last chance to get back into the game.
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  MacNiallais' two goals came at crucial times - particularly his 44th-minute palmed effort - and he also picked a further point from play
  Late choice Eoin McHugh also shone for Donegal as he kicked two scores while Frank McGlynn must also have been a contender for the man of the match award as he produced several charges into opposition territory following turnovers.
  Martin O'Reilly was another strong-running presence for Rory Gallagher's side.
  Donegal manager Rory Gallagher: "We were disappointed to lose Neil to a red card as he is one of our leaders.
  "I felt we should have been more than two ahead at half-time but we dug ourselves out of it and now plan ahead for meeting Monaghan in the semi-finals."
  Fermanagh manager Pete McGrath: "In the second half of the first half, we played exceptionally well. We had them on the back foot and we were playing with confidence and energy.
  "But Donegal's second goal punctured the life out of us."
  Donegal: MA McGinley; P McGrath, N McGee, E McGee; R McHugh, F McGlynn, A Thompson; R Kavanagh, E McHugh; L McLoone, M McElhinney, O MacNiallais; P McBrearty, M Murphy, M O'Reilly.
  Fermanagh: C Snow; M Jones, C Cullen, M O'Brien; D McCusker, J McMahon, B Mulrone; E Donnelly, A Breen; P McCusker, R Jones, R O'Callaghan; R Corrigan, S Quigley, T Corrigan.
  SUNDAY'S CHAMPIONSHIP RESULTS
  Leinster SFC quarter-finals
  Meath 0-20 1-13 Louth
  Westmeath 0-13 0-12 Offaly
  Munster SFC semi-finals
  Tipperary 3-15 2-16 Cork
  Kerry 2-23 0-17 Clare
  Connacht SFC semi-final
  Roscommon 4-16 2-13 Sligo
  SATURDAY'S RESULTS
  Leinster SHC semi-final
  Dublin 0-16 1-25 Kilkenny
  Christy Ring Cup relegation/promotion play-off
  Derry 1-14 1-21 Mayo
  Nicky Rackard Cup relegation/promotion play-off
  Fermanagh 0-12 7-19 Louth
target: The Ulster Football Championship finally came to life as two Odhran MacNiallais goals helped 14-man Donegal subdue Fermanagh in a fiery contest.