McMullan was the second person convicted after the withdrawal of Special Category Status for paramilitary prisoners, and he joined the blanket protest started by Kieran Nugent and refused to wear prison uniform. In 1978, after a number of attacks on prisoners leaving their cells to "slop out", the blanket protest escalated into the dirty protest, during which prisoners refused to wash and smeared the walls of their cells with excrement. As McMullan refused to wear a prison uniform he was not entitled to a monthly visit, and did not see his family until December 1979. McMullan described the visit in an interview: At his next visit, in March 1980, McMullan was expecting to see his mother Bernadette, but was instead visited by a priest who informed him of her death. During McMullan's imprisonment Bernadette had supported the protests and was involved in the Relatives' Action Committee, the predecessor to the National H-Block/Armagh Committee. In support of the protesting prisoners Bernadette took part in demonstrations across the country and abroad, and was part of a group of women who chained themselves to railings outside 10 Downing Street in London. McMullan became the longest-serving protesting prisoner when Nugent was released in 1980, and later in the year the protest in the Maze escalated further and seven prisoners took part in a fifty-three-day hunger strike. The strike was aimed at restoring political prisoner status by securing what were known as the "Five Demands":
The right to one visit, one letter and one parcel per week;
Full restoration of remission lost through the protest.
The strike ended before any prisoners had died and without political status being secured, and a second hunger strike began on 1 March 1981 led by Bobby Sands, the IRA's Officer Commanding in the prison. McMullan joined the strike on 17 August, after Sands and eight other prisoners had starved themselves to death. Following the death of Michael Devine and the intervention of the families of several prisoners the hunger strike was called off on 3 October, the 48th day of McMullan's hunger strike. He later described his feelings about the end of the protest: During his time in prison he studied with the Open University.
Life after prison
McMullan was released in 1992, and initially struggled to re-adjust to life outside prison. Since that time he has worked for Sinn Féin and helped set up groups for former prisoners. In 2000, he helped organise the transfer of the IRA prisoners' library of books from the former HM Prison Maze to Belfast's Linen Hall Library, with a view to setting up a lending library for former prisoners. McMullan has since worked as a political education officer for Coiste na n-Iarchimí, an umbrella organisation for Republican ex-prisoners groups, and, as of March 2007, he was working as a special advisor to Sinn Féin's Minister for EducationCaitríona Ruane.