The QPR stalwart won 52 caps for his country and was a member of Northern Ireland's World Cup side in 1986.
McDonald went on to manage Glentoran, winning the Irish League title in 2009, and also coached his country's under-21 side.
An Irish Football Association spokesman told Press Association Sport: "We are deeply shocked at the death of Alan, who won over 50 caps for Northern Ireland. To die at such a young age is terrible."
Former Northern Ireland manager Sammy McIlroy, a close friend and former team-mate of McDonald's, said: "I'm shell-shocked. Words can't even enter my head at the moment.
"Forty-eight is so young. Alan was always larger than life - during his career and after.
"I've just spoken to Norman Whiteside and he is devastated."
A short statement was also posted on the IFA's official website.
"The Irish FA is saddened by this morning's [Saturday morning's] news of the sudden death of Alan McDonald. Our thoughts are with his family at this time," it read.