But they produced an impressive response thanks to Romelu Lukaku's brace either side of Witsel's emphatic header in Bordeaux, earning them their first three points of Euro 2016 to significantly boost their chances of reaching the knockout stages.
Witsel, who was voted man of the match for his stellar performance in Marc Wilmots' midfield, told reporters afterwards that "the whole team put in a great performance."
He hailed Belgium's efforts throughout the first-half because, according to the 27-year-old, they "didn't give Ireland any chances" and "were constantly in their half" and "creating a lot of chances."
The Zenit St Petersburg anchor insisted that Wilmots' half-time team talk consisted of the squad talking about needing to "keep pushing forward" after the break, and he admitted that "the hardest thing" of the 90 minutes was "simply to get that first goal."
Witsel: Red Devils needed to quash criticism with win
Once Lukaku broke the deadlock, Belgium turned on the style as they romped to an impressive victory - their biggest in the tournament since 1970. A statement of intent of sorts, the demonstrated the attacking talent that Wilmots has at his disposal - and Witsel was pleased to see his compatriots thrive after stealing the lead.
"After that, we then managed to get a few goals," Witsel continued, saying that it was "what we [the team] needed after what's been a tough week" because they had "worked really hard" and were "both mentally and physically ready" for their clash with Martin O'Neill's hard-working Ireland.
He congratulated the entire Belgian team for putting in "a great performance" and said that it was always their aim "to go out there and take all three points."
Witsel called upon his teammates to "replicate this performance" in their final group game against Sweden in Nice next week, as well as hailing the fans for the "fantastic atmosphere" they helped create inside the Stade Nouveau de Bordeaux and saying that they "really helped" the team.
Belgium now need just a draw in order to guarantee their progression to the last 16, with Italy's head-to-head record making sure of the group's top spot. Should they secure second, Wilmots' men would take on the runners-up of Group F, containing Hungary, Iceland, Portugal and Austria.