Ms O'Brien (32) told the court she had been stopped behind Mr Flanagan's car at traffic lights in Walkinstown in June 2011. Her foot slipped off the clutch and her car tipped his.
Judge Matthew Deery, dismissing a €38,000 damages claim by Mr Flanagan against Ms O'Brien, said Ms O'Brien and the gardai involved had been unable to detect any damage to either car.
Awarding costs against Mr Flanagan (33), the judge told John Martin, for Ms O'Brien, he was of the firm conclusion Mr Flanagan was trying to make a large case out of an impact of little or no significance.
Mr Martin said Mr Flanagan had accepted an €18,000 settlement for injuries he received in another rear-ending incident in February 2010.
Ms O'Brien, of Inchicore, Dublin, said Mr Flanagan was upset and angry. She was frightened because he was shouting and intimidating her. When the gardai got out of their car they told him to "back off a little".
He had suddenly fallen to the ground on his knees complaining of pain in his back.
Mr Flanagan (33), an unemployed labourer, claimed he had been "violently thrown forwards and backwards by the impact" which injured his lower back.