The Premier League side dominated the Blades to secure their place in the draw for the fifth round.
McDermott praised his side's clinical finishing and insisted that they are not taking the FA Cup for granted this season.
"We're delighted to go through," said McDermott. "Second half, we were much more effective and clinical and the clean sheet was important.
"A number of Premier League teams went out and you can't take anything for granted. We're in the last 16 and delighted."
Reading are currently in 18thplace in the Premier League table.