[9] He had scored a goal in the quarter-final win over Naomh Bríd and was also central to the club's semi-final defeat of Aodh Ruadh.
[10] Brennan, playing under his future senior manager Declan Bonner, won the 2014 Ulster Minor Football Championship title with Donegal, scoring a late fisted goal and two points in the final against Armagh.
[11] He also played in Donegal's four-point All-Ireland minor final defeat to Kerry at Croke Park in 2014, scoring a point and also a goal at a critical stage in the game.
He set up an important goal for Lorcan Connor to finish in the final against Derry, dribbling the ball in a soccer-style run towards the opposing goalkeeper.
Brennan explained his thought process at the time as critical to the creation of the move: "There was a kick-out and it bounced once and I flicked it on, and everything opened up.
So I said 'if I pick this up, I'm going to fall straight on my face', so I kept going and then I was in on the goalie, nearly one-on-one, so a man came to me so I side-footed it over to Lorcan and he had an open net".
He scored a goal after 28 minutes against Monaghan during the 2018 National Football League fixture at St Tiernach's Park in Clones, as well as two points in the same game.
[24] Brennan started the final of the 2019 Ulster Senior Football Championship, which Donegal also won with Brennan's contributions including four points and a solo goal after a long run through the Cavan defence, momentarily appearing to lose the ball, only to boot it back past the goalkeeper and into the net.