Ronaldo, one of only three players not to be replaced by manager Zinedine Zidane after Wednesday's debacle in Germany, added to early strikes from James Rodriguez and Lucas Vasquez before Jese Rodriguez completed the La Liga rout before halftime.
The second period saw far fewer goalmouth incidents, although Ronaldo could have scored a second after being set-up by Vazquez.
Eibar then came close to notching a consolation when Borja Baston crashed a header against the bar.
"I'm happy with the way we played, more in the first half than the second," Zidane later told reporters in comments carried by the AFP news agency.
Barcelona's later 1-0 away defeat at Real Sociedad has also revived the hopes Real's slender title hopes, trailing by four points in third place with six games to play.
Zidane's team selection may have had more to do with Tuesday's quarterfinal second-leg with Wolfsburg than any retributive action for his side's tame two goal loss in Germany.
Gareth Bale, Toni Kroos and Marcelo, who came in for criticism for his on-field theatrics against Wolfsburg, began the match on the bench while Keylor Navas and Luka Modric were rested completely.
"We know what awaits us and what we are playing for on Tuesday. Now we just have to rest and prepare well," Zidane said.
"We aren't going to talk too much about tactics. We need to be really up for it and repeat the way we played today and like we usually have at home this season."
Barcelona had not won at the Anoeta since 2007 and that poor run continued Saturday, the home side going ahead after just five minutes through Mikel Oyarzabal's header after a cross from Xabi Prieto.
Lionel Messi had chances to score in either half with Sociedad's young Argentine goalkeeper Geronimo Rulli in superb form to deny him and Andres Iniesta.
It was the first time since Luis Enrique took over that Barca have gone three league games without a win and Atletico Madrid has closed to within three points in second after beating Espanyol 3-1.
Atletico's Fernando Torres canceled out Papa Kouli Diop's early strike for Espanyol before Antoine Griezmann and Koke sealed the points for Diego Simeone's men.
The top two in Spain meet again in midweek Champions League quarterfinal action with Barca leading 2-1 from the first leg in the Nou Camp, while Real Madrid has to pull back a two-goal deficit against Wolfsburg.