Michael O'Neill's side beat Norway 2-0 in their most recent match, a World Cup 2018 qualifier that moved them second behind Germany in Group C.
Wales dropped one place to 13th, England stay in 14th, while Scotland moved up eight spots to 59th.
Brazil went top for the first time in seven years, replacing Argentina.
The five-time World Cup winners became the first side to qualify for next year's tournament in Russia with a 3-0 victory over Paraguay last week.
Northern Ireland have climbed nine places up to joint 26th in Fifa's world rankings, level with the Republic of Ireland, who fell by two places.