England, who led 30-0 after 24 minutes in front of a record crowd of 24,375 in Huddersfield, will play Fiji in Hull on Saturday in their final Group A game.
"We hit a real purple patch in the first half," said McNamara.
"It is very difficult selecting a team and a 17 at this moment because there are so many very good players."
Ryan Hall scored a hat-trick as England ran in eight tries, six of them in the first half, to record their first win of the tournament.
Jonathan DaviesFormer Wales international and BBC Sport expert
"An exceptional first 20-25 minutes from England. They went forward and had excellent execution with great choices. They blew Ireland away and it was never a contest after that. They need to build towards Fiji now and hope not to have picked up any injuries."
McNamara added: "There are some players who are very disappointed to have missed out on selection.
"I have got to make sure that we navigate our best route to being successful and if that means some people miss out, then they miss out. I am not basing my selection on the basis of keeping everyone happy."
That Grand Final appearance had been O'Loughlin's only outing since he injured his Achilles in
the Challenge Cup final
at the end of August.
"Sean adds a whole lot to our team," added McNamara. "I thought he was very strong.
"He was tired at certain stages but we wanted him to get through the first 40 minutes and pushed him to do so. We gave him a break before getting him back out there. He is feeling pretty pleased with himself and so he should be."
McNamara said he was delighted his team prevented Ireland, who lost their opening game
32-14 to Fiji
in Rochdale, from scoring.
"Ireland put us under pressure at times but to nil any team at international level is a huge credit to the players. They showed some grit at times and kept the Irish out," he added.
Rugby League World Cup: McNamara happy with depth of squad
However he revealed two of his players had picked up injuries.
"George Burgess got a whack in the first five minutes and came off after 16 minutes, while James Graham is a slight concern but I don't think either are too major," said McNamara.
"They are dead-leg type injuries, lower-limb injuries, and given the scoreline and the state of the game we did not want to run any risks."
Skipper Kevin Sinfield explained that his team had adjusted their style of play in the second half as conditions deteriorated and the driving rain made handling difficult.
"It affected how we played," said Sinfield. "We had to tighten things up. At this time of year you have to expect these types of conditions in England. We have to be good enough to handle them."
Ireland coach Mark Aston was understandably disappointed with the scoreline and felt his team paid for periods of poor discipline.
"I am disappointed because for large spells of the game I thought we were very competitive," he said.
"But if you make mistakes and give penalties away then England will hurt you. At times we were asked questions and did not have the answer.
"I still think that there are areas where a team can exploit England."
Rugby League World Cup
Dates: 26 October to 30 November
Coverage: Watch England's group matches plus one quarter-final, one semi-final and the final live on BBC TV, online, mobiles and Connected TVs. Listen to every match live on BBC Radio 5 or 5 live sports extra
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