Celtic travelled to Spain’s Basque region to help Eibar mark their 75th anniversary but it was Griffiths who had most to celebrate.
The striker saw his penalty against Stjarnan saved in the first leg of their second-round qualifier after he came on as a substitute at Parkhead last week, but there was no repeat at the Ipurua Stadium as he converted with conviction from the spot after Efe Ambrose’s opener.
Griffiths then conjured up a wonderful strike out of nothing for his second before he seized on a slip by Andoni Errasti and slotted the ball into the net from a tight angle for his hat-trick.
Celtic dominated the match but their good work was almost undone midway through the first half when they conceded a penalty.
Darnell Fisher was caught out of position by a long cross-field pass by Eibar into the box and Dedryck Boyata blocked Dani Nieto’s shot with his hand.
The Eibar midfielder took the spot-kick himself but his effort was well saved by goalkeeper Logan Bailly, who was handed a first start by Celtic boss Ronny Deila. Eibar’s only goal, however, came from a penalty, Mikel Arruabarrena making no mistake in the 68th minute.