"It's necessary for our development," said Johnston whose team was shot out for a paltry 91 in the World Cup Super Eights match at Kensington Oval.
Australia had the match wrapped up by lunch after skipper Ricky Ponting won the toss and decided to give his bowlers a workout on a bouncy pitch.
Glenn McGrath (3-17) and Shaun Tait (3-39) made the match between the defending champions and the debutants a no-contest by removing the top four for 12 runs inside five overs.
Australia then stormed into the semi-finals by chasing down the target in just 12.2 overs.
"You learn with such experience, so with more exposure against high profile teams we can do better," said Johnston whose team now face Bangladesh here on Sunday and Sri Lanka on April 18 in Grenada.
"Up until against Australia we did well. We knew that Australia would be a big challenge. We must take one positive in that we played the best sides in the world and have learned something."
Ireland dumped Pakistan out of the World Cup with a three-wicket upset win after playing a tied game against Zimbabwe in the group stages.
"I knew it was going be tough. Australia are not two-time world champions going for a third for nothing," said the Australian-born Johnston who took the only wicket to fall when he clean-bowled Adam Gilchrist.
"We have been outclassed by a world class team. We wanted to be competitive but it didn't turn out that way. Unfortunately such starts don't allow you to be competitive and they have been the best team for the last ten years.
Johnson hoped his batsmen learn from facing quality bowlers like McGrath and Tait.
"They were great. It was nice to play quality bowlers like they are. Our top four couldn't get runs," said Johnston, one of only three batsmen to reach double figures with 17.
John Mooney top-scored with 23, while Kevin O'Brien made 16.
Ireland's South Africa-born coach Adrian Birrell conceded Australia provided the toughest test.
"They got into us right from the beginning. They bowled well and our top order got us into trouble and we could never get out of it and lost badly," said Birrell.
Birrell felt for the crowd whose day out ended before lunchtime.
"We wished we could have done better for them and given them better entertainment."