Sullivan, formerly of Sunderland, played in a summer friendly for Doolin when the former Shelbourne captain was still in charge of UCD last June.
20-year-old Whelan, also a product of Doolin’s Belfield production line, is the son of former Bohemians and Shamrock Rovers’ favourite Paul Whelan and strangely enough is following in his grandfather Ronnie senior’s footsteps as the second member of the family to line out in United’s claret and blue.