"It was tough enough winning the Heineken Cup last season but I guess it is going to be even tougher defending it - we certainly won't have any element of surprise on our side this time," said Cullen.
"The squad is desperately hungry to move on and build on the success of last season but the standard of teams simply gets better every year and the French teams, in particular, have recruited very heavily and will be a massive challenge.
"London Irish are a very strong physical side with a number of powerful runners, superbly coached by Toby Booth.
"They will be a real handful and a huge challenge but, as we also like to play a brand of attacking rugby, it should be some match."
Recalling last season's campaign, the big lock added: "We were a bit ragged getting out of our pool last season and it wasn't always the prettiest.