Town's defensive problems last term were well documented as the club conceded 71 goals in total, more than several clubs who finished below them in the table.
Shirtliff, appointed on Monday, told the club's official website: "Between myself and the rest of the coaching team we will be looking to improve the defensive record.
"You can have the flair and the attacking side of the game, Danny and I love all that, but you have to keep the old back door shut.
"That's something we will be working on, which will get started on Monday. If the players knuckle down then they will enjoy the training."
He added: "I'm pleased to be here and the opportunity to work with Danny again is something I'm looking forward to. While we are friends, we are professional people and have our own opinions, which is healthy."