She’s a leading candidate to be New York’s next mayor. She’s already one of its top Irish-American officials. Christine Quinn is also a lesbian and proud of it. And that’s why the City Council speaker won’t be marching in Saturday’s St. Patrick’s Day Parade, an event so entrenched in New York tradition that it’s older than the United States. Quinn’s rising political prominence is bringing a decades-long dispute between parade organizers and gay activists back into sharp relief.