After beating Hungary in their opening Group 1 game, James Scott's squad knew that a win in this fixture would see them advance to the next stage before facing hosts Norway on Friday.
Three first-half goals did the job. Orlaith O'Mahony lobbed the goalkeeper on 26 minutes, Allie Long slotted in from close range on 38 minutes and Abbie Larkin converted a penalty on 45 minutes.
Everything flowed the way of the Girls in Green in that first period with their fitness and movement proving too much for Bulgaria, who were often locked inside their own penalty area defending.
Bulgaria struggled to patch together any moves and failed to register a single shot on the Irish goal with Peamount United's Claudia Keenan given little to do.
The stylish play of Scott's team - as evidenced in both games so far - means that they will now be seen as a real contender to reach the finals tournament in Bosnia & Herzegovina. But there is still work to be done.
The focus will now turn to Friday's clash with Norway, which will be a battle to see who finishes top of Group 1.
Republic of Ireland: Keenan; Russell, Fleming, Mackin, O'Hanlon (J Loughrey 46); O'Brien, O'Mahony (Kelly 61), Lawrence (Ralph 81); Long, Larkin (O'Sullivan 46), O'Leary (McCarn 61).
Bulgaria: Kabova; Raycheva (B Demirova 71), Stankova, Ilieva; M Mihaylova (Trifonova 86), R Mihaylova (P Demirova 46), Angelova, Metodieva (Vasileva 78); Cwetkowa (Genova 46), Sergeeva, Yordanova.
Referee: Alina Peşu (Romania).
UEFA Women's Under-17 European Championship Qualifying Round - Group 1
Saturday, October 9 | Republic of Ireland 3-1 Hungary | Sandnes, Norway
Tuesday, October 12 | Republic of Ireland 3-0 Bulgaria | Sandnes, Norway
Friday, October 15 | Republic of Ireland v Norway | KO TBC | Sandnes, Norway