Murtagh is suffering from a stress fracture in his right foot and, therefore, has been ruled out of the tournament.
The 29-year-old Sorensen has played eight ODIs for Ireland in which he has scored 97 runs and taken 12 wickets. Sorensen’s last ODI appearance in Ireland colours was against Scotland in Dublin in September 2014.
Any injury or illness-based replacement requires a written submission to the Event Technical Committee along with a diagnosis from a medical practitioner as to the extent of the injury or illness. Once replaced, a player may not return to the squad save as an approved subsequent replacement for another injured or ill player.
Ireland is in Pool B of the ICC Cricket World Cup 2015 along with defending champion India, former winners Pakistan and West Indies, African rivals South Africa and Zimbabwe, and qualifier United Arab Emirates (UAE). Ireland will play its opening match against the West Indies at Saxton Oval in Nelson on 16 February.