The 20-year-old centre-half has signed a two-month deal at Valley Parade and will go straight into manager Phil Parkinson's squad for Tuesday night's home game with Chesterfield.
Parkinson, who recently lost central defenders Luke Oliver (Achilles) and Andrew Davies (knee) to season-ending injuries, told the club's official website: "He's competitive and looks both decent in the air and on the ground. I like what I've seen of him.
"We've done our homework on him and we've heard that his commitment and desire is first-rate. Whenever we bring a player into the club, we always talk about getting the right characters in. John certainly seems to fit that bill.
"He's someone who is going to have a real go for us. I think he will blend in really well with the players we already have in the squad."
Egan, 6ft 2in, has been capped at under-17, under-19 and under-21 level for the Republic of Ireland and joined Sunderland from hometown club Cork City in 2009. He has yet to appear in the first team at the Stadium of Light and has had loan spells at Sheffield United and Crystal Palace this year.