Donald I (a.k.a. Domnall mac Ailpín) lived from 812 to 863 and was King of the Picts and Scots from 858 to 863. The wider picture in Scotland at the time is set out in our Historical Timeline.
Donald made less of an impression than his brother. He was described as wanton and you get the sense a playboy prince, a younger son who never expected or perhaps even wanted to become King. He only reigned for five years. He died in Perthshire (accounts differ as to whether in battle or of natural causes) and was buried in the graveyard at Saint Oran's Chapel on the Isle of Iona.
King Donald I's main contribution was to complete the task begun by his brother of suppressing any further challenge to the House of Alpin from the Pictish nobility for the Pictish Crown. He also oversaw the introduction of a set of laws known as the Laws of Aedh (or Aed). These included the law of tanistry, under which the successor of a king was elected during his lifetime from members of his family: often a brother or cousin rather than a son. The aim was to ensure that in an age when very few men died old, the succession would pass on to someone old enough and able enough to handle it.
Donald's successor, Constantine I of Scotland, was elected to succeed him by this method. Constantine was Donald's nephew, and the son Donald's predecessor, Kenneth I. Tanistry governed questions of succession to the Scottish Crown until the reign of Malcolm II of Scotland.