Ireland have beaten Bulgaria 2-1 on the night, 4-2 on aggregate, to secure their place in League B of the UEFA Nations League.
Hallgrimsson's team were the better side on the night and across the two legs of the promotion/relegation play-off.
The national team will now play two friendlies in June - against Senegal and Luxembourg - before starting their World Cup qualification campaign in September.
Ireland take the lead through Adam Idah.
The Celtic striker scored at the back post, turning home Mark Sykes' cross following a break by Ireland.
That goal should be enough to cement Ireland's place in League B, with the team now leading 4-2 on aggregate.
Excellent goal by Evan Ferguson.
The Ireland striker plays a one-two with Finn Azaz on the edge of the penalty area, and finds space in the box, before hammering the ball into the roof of the net.
His fifth goal on his 20th appearance, the striker was about to be substituted before the goal.
The second half has started after a delay of around 12 minutes or so. The referee, Umut Meler of Turkey, picked up an injury in the first half and can't continue. Mehmet Türkmen, his compatriot and the fourth official, has replaced him.
The visitors have taken the lead against the run of play.
Ireland have been the better side and looked most likely to break the deadlock.
But Bulgaria have hit them with a sucker punch.
From a corner, the ball lads with Antov outside the six-yard box and he scuffes it over the line.
A poor goal to concede from Ireland's perspective.
Ireland have been the superior team in the opening 10 minutes.
The home side have dominated possession (73%), pinned Bulgaria back into their half, and created a few openings.
Brady's crosses from left back, in particular, have been causing Bulgaria issues.
HallgrÃmsson has made two changes to the team that won on Thursday night.
In central defence, Jake O'Brien replaces Dara O'Shea.
While upfront, Evan Ferguson starts as Ryan Manning drops out of the team.
Nathan Collins captains the team again, while Matt Doherty will earn his 50th cap.
Ireland: Kelleher; Doherty, O'Brien, Collins (C), Brady; Johnston, Cullen, Knight, Azaz, Parrott, Ferguson.
Bulgaria: Plamen Iliev; Nikolay Minkov, Ilia Gruev, Fabian Nuernberger, Valentin Antov, Bozhidar Kraev, Kiril Despodov (C), Anton Nedyalkov, Simeon Petrov, Marin Petkov, Georgi Milanov