Leinster have previous with the Castres, having beaten them twice in last season's European Cup and squared off with Harlequins in 2009 on their way to European glory.
And, with a trip to Twickenham Stoop and Adams Park ahead for Leinster, Cronin wants London's Irish to come out in force.
"Usually there's only one group of death, but they all look tough pools," he said.
"We've got two trips to London which all the boys will be looking forward to.
"Hopefully there are a few Irish living in London who fancy coming along and giving us their support, as they did at Northampton last season.
"We were disappointed with ourselves last season. We work so hard to progress from a pool that including Northampton, Castres and Ospreys and then felt we let ourselves down in the quarter-final at Toulon.
"Leinster have a proud tradition in Europe and it will be a huge focus for us next season."
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