Referring to Bloodgate at Harlequins and the drugs scandal at his old club Bath, Borthwick said: "It was vital to get back to real rugby.
"It was a tight game, a classic. There was excitement and a wonderfully close finish. It was rugby at its best."
Sarries, ninth last season, outlasted Irish - runners-up in the play-off final here at Twickenham in May.
Stand-off Glen Jackson scored the first try of the Premiership season after 21 minutes from a tap-penalty.
Twelve minutes later, he spotted an area to cross-kick for Andy Saull to score.
The Exiles' only try came at the start of the last quarter through George Stowers.
But fly-half Ryan Lamb failed with three of his six kicks as Irish - down 18-3 at the break - fell short.
And it was not a happy return to HQ for Irish full-back Delon Armitage, who was sin-binned when ref Dave Pearson told him: "You ran 40 yards to join a scrap."
SCORERS - Saracens: tries Jackson, Saull; con Jackson; pens Jackson 2. London Irish: try Stowers; pens Lamb 2, D Armitage.
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