O'Neill's side are guaranteed at least a play-off place after beating world champions Germany but a win in Warsaw will secure automatic qualification.
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"To beat the world champions is a sensational effort," O'Neill said after Thursday's 1-0 win in Dublin.
"We have reached the play-offs and we can go for it on Sunday now."
While victory against Poland will ensure the Republic reach a third European Championship finals, defeat will mean they would have to qualify through the play-offs.
A draw would leave both Poland and the Republic on 19 points - and as to who would clinch the second automatic place (assuming Germany do not lose to Georgia), with head-to-head records the top tiebreaker:
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The Republic produced a heroic defensive display to condemn Germany to their first competitive defeat since October 2014.
The winning goal came from a wonderful finish by substitute Shane Long.
"The players put in a fantastic effort," added O'Neill.
"We showed bravery and courage. We had to withstand a lot of pressure, which you'd expect, and we did that."
Republic of Ireland manager Martin O'Neill says his players will "go for it" when they visit Poland in Sunday's final Euro 2016 qualifier.