O'Neill enjoyed a dream start to his Ireland reign as Robbie Keane sparked a demolition of Latvia at the Aviva Stadium in Dublin.
Keane extended his senior international goal tally to 62 just 22 minutes into O'Neill's first game at the helm, prompting namesake and assistant manager Roy Keane to punch the air in delight before returning to his seat on the bench.
McGeady, who prospered under the Ulsterman during his time in charge at Celtic, extended the lead a with a well-taken second 22 minutes from time, just his third goal in 62 senior appearances for his country.
There was more to come and substitute Long capped a convincing performance with a 79th-minute third as the Latvians wilted in the face of a concerted assault.
O'Neill made six changes to the side sent out by Noel King in last month's final World Cup qualifier, a 3-1 victory over Kazakhstan, and Ireland, ranked 60th by FIFA, were simply too strong for 117th-placed Latvia.
Indeed, had it not been for keeper Andris Vanins, the lead could have been significantly greater by the time the half-time whistle sounded.
The presence of playmaker and Wes Hoolahan in O'Neill's starting line-up, as well as that of wingers McGeady and James McClean, was a statement of intent, and one which was warmly welcomed by the home supporters.
The two wide men ran repeatedly at the Latvia defence, while Hoolahan exploited the space in between the visitors' defensive and midfield lines to good effect as the Republic dominated.
They might have been ahead with just 12 minutes gone when McGeady intelligently squared a free-kick to Hoolahan to help it across goal to Keane, who uncharacteristically shot straight at keeper Vanins.
But the one thing Trapattoni was able to rely upon most consistently throughout his five and a half-year reign was the Los Angeles Galaxy frontman's ability to find the back of the net.
The 33-year-old was in exactly the right place at the right time to stab home from close range after McClean had flicked on McGeady's 22nd-minute corner, and he might have doubled his tally within two minutes had Vanins not produced an important save.
McClean dragged a 34th-minute wide of the left post and then sliced another wide of the right in the final minute of the half with Ireland in almost complete control.
To their credit, Latvia refused to lie back and accept their fate and went close within seconds of the restart when first-half substitute Aleksandrs Fertovs sent a dipping long-range effort over Keiren Westwood's crossbar.
But it was the home side who continued to pose the greater threat and an off-balance McClean blazed over after Hoolahan had pulled the ball back to him.
Vanins managed to beat away McGeady's 63rd-minute strike, but with his defenders dropping ever deeper, he was seeing for more of the ball than he would have liked.
But the Spartak Moscow man was not to be denied and he got his reward when he thumped a skidding right-foot drive into the bottom corner from 25 yards to make it 2-0.
The victory was complete 11 minutes from time when Long tapped home Seamus Coleman's cross at the far post just six minutes after his introduction.
Fellow replacement Anthony Stokes drew a flying save from Vanins at the death, but the home fans filed out of the stadium more than content.
|Republic of Ireland||Team Statistics||Latvia|
|1||1st Half Goals||0|
|10||Shots on Target||0|
|8||Shots off Target||2|
|67.6||1st Half Poss.||32.4|
|64.6||2nd Half Poss.||35.4|