Whelan is born into a family of footballers. His father, Ronnie Whelan, Sr., is an Irish international and a key member of the successful St. Patrick’s Athletic F.C. of the late 1950s and early 1960s. His brother, Paul Whelan, plays for Bohemian F.C. and Shamrock Rovers F.C..
Whelan is a skillful and industrious midfield player who, after an unsuccessful trial period for Manchester United F.C., makes his League of Ireland debut for Home Farm F.C. on his 16th birthday at Tolka Park.
Whelan is signed for Liverpool F.C. by Bob Paisley for a bargain £35,000 on September 19, 1979, a few days before his 18th birthday, and makes his debut eighteen months later, on April 3, 1981. He goes on to become an integral part of the dominant Liverpool team of the 1980s. He remains with the club until 1994. In 100 Players Who Shook The Kop, a poll of 110,000 Liverpool fans conducted by the official Liverpool F.C. website, Whelan is in 30th position.
Whelan plays for the Republic of Ireland national football team at one UEFA European Championship (1988) and two FIFA World Cups (1990 and 1994), turning out a total of 53 times for the national side between 1981 and 1995.
Whelan finishes his career at Southend United F.C., where he is also player-manager through the 1996-1997 season. He later manages in Greece and Cyprus, with Panionios G.S.S., Olympiakos Nicosia and Apollon Limassol.
Since retirement Whelan has begun a media career and is a regular contributor to RTÉ Sport in Ireland. He is also Patron of the Myasthenia Gravis Association in Ireland where he is an active fundraiser and works to raise awareness for this rare autoimmune disease.