Citroen World Rally Championship driver Breen - this year in a Citroen DS 3 R5 for the Circuit after winning for the works Peugeot team last season - took three stages to get into the grove.
He hit the front on Friday’s titanic 19-mile The Glens stage taking in the Northern Irish coast.
“That’s the best piece of Tarmac in the world,” Breen told Autosport at the finish. “This is what Irish rallying is about.”
Kajetanowicz - who was also Breen’s closest challenger last year - benefited on the final day as Breen chose super-soft tyres anticipating wetter stages.
The heavier rain never came and Kajetanowicz’s Pirelli-shod Ford Fiesta R5 worked better in the conditions.
By the last service he had eroded the lead down to 10.4 seconds from 25.1s on Friday evening, but the service allowed Breen to change tyres and he began to pull away again.
A rock on SS12 caused a puncture for Breen, bringing the gap back down with two stages left, but by the end of the next stage he had taken 6s out of Kajetanowicz and the winning margin at the finish was 10.6s.
“I’m not giving this bloody rally away,” Breen said at stage end.
The event was also a round of the reformed British Rally Championship, and the title favourite Elfyn Evans was fastest at first, 12.4s up overall after two stages.
Then a pulley in the alternator gave way on SS3 after power steering problems had hindered him, and his run of three wins from three rallies - including WRC2 victories - was ended.
It was an even more abrupt end when he hit a wall on stage 12, Hamilton’s Folly, after returning for day two.
Josh Moffett was instead the BRC victor in fourth overall, behind Alister Fisher who rounded out the podium.
The young Irishman is the nephew of legendary Irish Tarmac driver Bertie Fisher and is contesting that championship this year.
Breen’s David Greer Motorsport team-mate Jonny Greer and Skoda Fabia driver David Bogie rounded out the BRC top three and overall top six.
Lukas Pieniazek topped the ERC Juniors as the works Opel factory team suffered mixed fortunes.
Chris Ingram was second - the Brit in his first year with a works backed team - but had struggled in his first time with the car on Friday. Though Saturday was stronger, the gap to Pieniazek was too great.
Marijan Griebel, who won this event in the Juniors last year - had a puncture on Hamilton’s Folly that relegated him out of the running.
Irishman Robert Duggan topped the British juniors ahead of Vauxhall Adam R2 team-mate Mattias Adielsson.