Tuilagi has recovered from the ankle injury which kept him out of England's 38-18 win against Scotland and he is now competing with Brad Barritt and Billy Twelvetrees for a start in Dublin.
The onus is on Tuilagi to oust one of them from the side - and Tuesday was D-day, with England holding a decisive double training session at their Surrey headquarters.
It is always difficult but it is a great headache to have," said England skills coach Mike Catt.
"From a team point of view you have three guys and each of them bring special attributes. It is great Billy has come along and performed the way he did against Scotland on his debut.
"Manu has been out of the game for three or four weeks. We will see how they all train and we will go accordingly from there.
"If the guys have performed exceptionally well on the weekend, the guys coming in need to make sure they have the tempo and up the intensity to really put their hands forward.
"That is what we want on Tuesday."
Barritt runs the England defence, even from the less familiar outside centre role he occupied in Tuilagi's absence last weekend.
Catt outlined how important that would be against an Ireland side containing Jonathan Sexton and Brian O'Driscoll.
"It would be very hard (taking him out of the equation)," Catt said.
"O'Driscoll is playing exceptionally well at the moment. I thought that first half Ireland produced (against Wales) was exceptional. We know they are capable of doing that.
"Defence is across the board. Everybody knows their roles.
"We need to focus on our attacking game too, which went pretty well against the Scots, and try and starve them of the ball so they can't do what they are good at.
"It is a little game of chess we have to try and win."
England have not yet officially ruled out Ben Morgan from their Dublin plans after the Gloucester number eight sprained his ankle early in the second half against Scotland.
But Morgan's chances look bleak. He was wearing a protective boot and walking at the England hotel with the aid of crutches.
James Haskell is set to start in Morgan's anticipated absence with Tom Wood shifting across the back row from blindside flanker to number eight.
The Wasps number eight Billy Vunipola has returned to training and he would offer England a powerful and dynamic option off the bench.
"Billy Vunipola trained with us this morning and he is doing the afternoon session. I am looking forward to seeing what he can bring," Catt said.