On 9 September, 1342, he was married to Elizabeth de Burgh, who as a ten-year-old was six years his senior. The year after Elizabeth was born, her father died. He had been the Earl of Ulster, and marrying Elizabeth allowed Edward III to grant Lionel that title as early as 1347; Lionel also gained possession of vast estates in Ireland. In 1352 the couple lived together as husband and wife, aged 14 and 20 respectively. They had one daughter, Philippa, born 16 August 1355.
While Edward was heading to the continent as part of the Hundred Years War, he appointed Lionel as his representative in England in 1345 and 1346. Lionel was old enough to join his father and brothers on military campaigns to France in 1355. He grew to be about 6'10" tall, and would have been an imposing figure on the battlefield.
Lionel was present in 1360 for the signing of the Treaty of Brétigny, during which time he sent his page, Geoffrey Chaucer (then in his late teens) back to England with letters (discussed here). With the war with France "settled" (little did they know), Lionel went to Dublin in 1361 to be governor of Ireland. The following year, Edward created a dukedom and named Lionel Duke of Clarence. Edward also hoped to make Lionel King of Scotland, but that was less successful even than trying to control Ireland. In 1366, the Statutes of Kilkenny made an attempt to control some of the issues in Ireland that were disturbing to England.
Prior to this, however, tragedy struck with the death of Elizabeth in 1363. This left Lionel open to another advantageous marriage. This time, ties with Italy were sought through marriage to Violante Visconti. Arrangements had been made earlier by, among others, Geoffrey Chaucer, who traveled to Italy (mentioned previously here); the marriage itself was mentioned here.
Lionel and Violante were married in June 1368, kicking off months of festivities. Lionel took ill and died 17 October of the same year. His father-in-law, the ruthless Galeazzo II Visconti, was suspected of poisoning him, but it could not be proven. Visconti wanted the alliance with England, and Violante was not going to inherit anything of value with the death of Lionel, so there is no rational reason for Lionel's father-in-law to have him killed.
His body was buried in Pavia, but later returned to be laid to rest beside his first wife at Clare Priory in Suffolk. His daughter, Princess Philippa of Clarence, inherited the earldom of Ulster.
What was the perceived need for the Statutes of Kilkenny? It addressed the concern about English residents of Ireland becoming "too Irish." I'll tell you about that next.
