Mac Niallais won the 2018 Donegal Senior Football Championship, scoring eight points in the final to help Gaoth Dobhair to their first Donegal senior championship since 2006, and being awarded man of the match. Mac Niallais then starred in Gaoth Dobhair's first ever Ulster Senior Club Football Championship-winning campaign later in 2018. In the Ulster semi-final defeat of Crossmaglen Rangers at Healy Park in Omagh, Mac Niallais scored four points, including one free. In the final against Scotstown, Mac Niallais was again awarded the "Laoch na hImeartha" after scoring four points, including three frees.
Inter-county
Mac Niallais was first called into the Donegal senior team as an 18-year-old in the winter of 2011 by manager Jim McGuinness. He played in the 2012 Dr McKenna Cup but injuries hampered his progress that year for club and county. He returned to training ahead of the next season and played in the 2013 Dr McKenna Cup. During that competition, he scored an early goal against St Mary's in a one-point win at MacCumhaill Park. In the 2013 National Football League he made substitute appearances against Down in the second game and against Dublin in Donegal's final league game of the season, both in Ballybofey. He also played in the under-21 team that lost to Cavan in the 2013 Ulster final. Mac Niallais arrived in 2014 having played around a half-an-hour of meaningful senior inter-county football for Donegal. That year would bring his first league start -agaiainst Laois at O'Moore Park. He palmed home a goal in the twelfth minute and scored a point later in the same match. The year also brought games in the Senior Championship for Mac Niallais for the first time. Called onto the field when Donegal were forced to make emergency reparations to the midfield following injuries to Rory Kavanagh and Neil Gallagher, Mac Niallais scored four points and collectedthe man of the match award in the Ulster semi-final against Antrim at Clones. He then won his first Ulster senior title, helping himself to three points in the final against Monaghan. Under the management of Rory Gallagher, Mac Niallais scored a brace in the seven-point 2016 Ulster Senior Football Championship quarter-final victory over Fermanagh. He later left the Donegal team. He headed stateside. Mac Niallais returned to the Donegal team under the management of Declan Bonner in 2018. He started the final and scored a point as Donegal secured the 2018 Ulster Senior Football Championship. Following the death of a clubmate in a car accident in January 2019, Mac Niallais opted out of the Donegal panel that year. He thus missed out on the 2019 Ulster Senior Football Championship, which Donegal won. He also opted out of the Donegal panel in 2020.
