The Tigers looked on course for a big total after an electric start from opener Tamim Iqbal but some tight bowling and regular wickets saw Ireland keep the tournament co-hosts down to 205 all out.
That was a total which could realistically be passed — albeit on an extremely slow pitch — but scores of 38 and 37 from Niall and Kevin O’Brien were the best they managed and they were bowled out for 178.
Reflecting on the defeat, Porterfield said: “It’s not hard to work out where we lost this game. We let ourselves down with the bat.
“After 50 overs it was pretty good and the way we hauled it back after the first five or six overs in the field was really good.
“You can’t afford that many soft dismissals in this tournament but we have to bounce back from this, we can’t let it bother us. We have to leave the batting here and move on to Bangalore (where they play England on Wednesday).”
Tigers skipper Shakib Al Hasan said he played a back-seat role at the interval but was impressed with the self-belief in the home dressing room after posting a modest total.
“I didn’t say anything (to the team). Everyone else was saying we will win by 20 runs so I had that belief in them,” he said.
“They showed their character. It was a really hard game for us. The ball wasn’t doing much but our bowlers did a good job and the fielders, for them it was a tremendous game.
“It’s going all right now, but it was a very good game. I thought we played really hard cricket. Although we didn’t bat well, our bowlers and fielders showed great character and support for us. It’s good to have support like this and hopefully we can improve ourselves from here.”
Shakib also praised the bowling of Shafiul Islam, who claimed four wickets in a decisive second spell.
“He was expensive early on but we always knew he could come back and do the job for his team,” Shakib added. “I thought he swung it really well. He put it in the right areas and changed the game for us.”
Meanwhile, Australia captain Ricky Ponting saluted his pace bowlers for setting up a comprehensive seven-wicket victory over New Zealand in their World Cup Group A encounter in Nagpur.
Mitchell Johnson picked up four for 33 and Shaun Tait claimed three for 35 to dismiss the Kiwis for a moderate 206 with nearly five overs to spare.
In reply, openers Shane Watson (61) and Brad Haddin (55) put on 131 from just 109 balls to put the champions on course for a comfortable victory, which was achieved in the 34th over with seven Australian wickets intact.
Ponting credited his fast bowlers for bringing up their second straight win of the tournament and singled out Johnson for special praise.