It has been a remarkable league campaign for Linfield, who ended Larne's two-year stint as champions in emphatic fashion.
As their rivals faltered, Linfield consistently collected points and became the first team to seal the title before the split - which occurs with five matches remaining.
The victory over Glentoran, who were unbeaten against the Blues this season in five outings, was the icing on the cake.
In a frantic first half, Morrison opened the scoring as he squeezed an effort past Glens goalkeeper Daniel Gyollai in the ninth minute, but Declan Devine's side hit back when Jordan Jenkins fired home in the 22nd minute.
However, Linfield stalwart Millar restored Linfield's lead on the half-hour and the hosts, who spurned chances to extend their lead after the break, never looked back.
Such was their dominance, Linfield's lead at the full-time whistle over Glentoran stood at a mammoth 25 points.
It is a 57th Gibson Cup for Linfield, and no team has won their domestic league more times in world football than the Blues.