In 2021, Halliday was awarded the James Spence Medal for research into neonatology, for coordinating two of the largest neonatal multicentre trials for prevention and treatment of a number of neonatal respiratory illnesses and for a breakthrough in the development of a new lung surfactant[1][2] that brought relief to very small babies suffering from infant respiratory distress syndrome (RDS).

[3] After his postgraduate training in Belfast, he moved to the United States where he spent three years, first working at the Rainbow Babies & Children's Hospital in Cleveland and later at the Cardiovascular Research Institute, University of California in San Francisco.

[3] In 1980, he passed the MD Thesis at Queens University with a paper on the role of left to right shunting through a patent ductus arteriosus.

As the control group was insufficiently large, Morley suggested to Halliday that he make contact with the Swedish physician Bengt Robertson who had also developed a surfactant Poractant alfa known under the trade name Curosurf.

[9] Halliday spent a month with Robertson at the Karolinska University Hospital in Stockholm in November 1984, when they ran a 27 day pre-clinical trial both In vitro and in rabbit fetus's ventilated with a plethysmograph where Turfsurf was compared to Curosurf.

[9][15][16] Halliday witnessed the highly efficacious Corusurf at work when pre-term twins were treated, who were going blue in the face due to a lack of oxygen and almost immediately after treatment, turned a healthy pink.

[4] In 2000, Halliday coordinated a large multicentre, randomized clinical trial of early corticosteroid treatment, known as OSSECT to compare whether there was any difference in early (<3 days) with late (>15 days) steroid therapy and dexamethasone with inhaled budesonide treatment in preterm infants at risk of developing chronic lung disease.

[22] In 2010 Halliday was awarded the Maternité Prize from the European Society of Perinatal Medicine for his advocacy in international medical politics that led to structural reform.