Having beaten Poland and narrowly losing to Russia, Vera Pauw's team were moved to the Pinatar Arena for this Play-Off game that Ireland deservedly won.
Inside the opening five minutes, Kyra Carusa was presented with a fantastic opportunity to break the deadlock when tipping the ball past Wales goalkeeper Olivia Clark but her shot from an angle hit the side netting.
Carusa would go close again, this time with a header when meeting a corner kick, while Wales star Jess Fishlock saw an effort from distance blocked by Niamh Fahey.
On 25 minutes, O'Sullivan was quickest to react to a loose ball on the edge of the area and she smashed in a superb strike to record her 17th senior goal.
Niamh Fahey should have made it 2-0 just before the interval when she found herself unmarked at a corner kick but the Centurion headed over the bar.
Teenager Abbie Larkin replaced Carusa at the start of the second period and it was more of the same from Ireland as they rolled out their controlled game-plan - working on both sides of the ball.
Wales tried their luck from distance on 55 minutes when captain Sophie Ingle unloaded a shot but it clipped the crossbar and went over, not troubling goalkeeper Courtney Brosnan.
Just past the hour mark, Ireland introduced goalkeeper Eve Badana for her fourth senior appearance and first game since 2013.
Pauw's team looked sharp on the attack with McCabe fizzing a shot wide and Larkin taking aim from around 25 yards out. Then Lucy Quinn headed over 84 minutes from inside the penalty area.
This was the type of performance that Pauw wanted from her players and it should aid their preparation for their next FIFA Women's World Cup Qualifier away to Sweden in April.
Wales: Olivia Clark; Charlie Estcourt, Rhiannon Roberts, Gemma Evans (Anna Filbey 60), Rachel Rowe (Lily Woodham 60); Ceri Holland (Elise Hughes 70), Sophie Ingle, Angharad James, Ffion Morgan (Kayleigh Green 70); Jess Fishlock (Helen Ward 70); Natasha Harding (Georgia Walters 60).
Republic of Ireland: Courtney Brosnan (Eve Badana 63); Niamh Fahey, Louise Quinn, Savannah McCarthy; Jess Ziu, Ruesha Littlejohn, Denise O'Sullivan, Katie McCabe; Heather Payne (Leanne Kiernan 63), Lucy Quinn (Amber Barrett 85); Kyra Carusa (Abbie Larkin 46).
Referee: Bockova Miriama (Slovakia).