Donal was raised in a sporting background. His father, Gerald Lenihan, was an All-Ireland heavyweight boxing champion and Gaelic footballer of distinction, and played in the same team as Jack Lynch. Donal attended second level school in Christian Brothers College, Cork. He captained his school to Munster Junior and Senior Schools titles and was also captained for Irish schools
He was a student at UCC and played for the rugby team while studying there.
Lenihan played his first test match for Ireland on 21 November 1981 versus Australia at the age of 22. Famous for his aerial skills in the line-out, the hard-nosed second row was ever present in the Irish team for over a decade which saw two Triple Crowns and three 5 Nations Championships victories. He is best remembered for his break which set up Mike Kiernan's championship clinching drop-goal against England in 1985.
He played four matches in the inaugural 1987 Rugby World Cup where he was the Irish captain and played three matches in the 1991 Rugby World Cup. Lenihan, a proud Munsterman, captained his country 17 times. He was selected for three Lions Tours (1983, 1986, 1989) and was the popular captain of the unbeaten 1989 midweek side that became known as Donal's Donuts. His last cap came against Wales on 18 January 1992.
After retirement from rugby he took over as manager of Ireland in 1998 after the resignation of Pat Whelan. He resigned as manager at the end of the 2000 season to take over management of the British and Irish Lions for their 2001 tour. He appears regularly as a co-commentator on radio and TV for rugby matches and writes for the Examiner. He works as a financial consultant in Cork.