Millwall are spending their pre-season in the Republic of Ireland, where their opponents are already midway through their domestic season.
The Lions beat Airtricity First Division leaders Shelbourne 3-0 on Monday and will be travel to fifth-placed Longford Town on Wednesday.
Lions boss Kenny Jackett told Millwall's official website: "The Shelbourne and Longford players are very hard working and committed in their approach, which gives us just the test we need at this stage of our preparations.
"They are in the middle of their league season at the moment which means their fitness levels are good.
"I was pleased with the way we matched Shelbourne in that respect on Monday and I am looking for a repeat of that in the second game."