O'Neil joins as contracts director, bringing over 25 years' experience in construction and civil engineering.
While at Farrans, O'Neil was responsible for around £100m of project annually. He was involved in the Edinburgh Tram extension and the Northern Spire Bridge in Sunderland, as well as a series of major wind farm projects in England, Scotland and Wales.
In his new role, he will take responsibility for delivering offsite projects.
Mark Lowry, managing director of The McAvoy Group, said: "We are continuing to grow and develop the McAvoy business, which in turn means strengthening the team with the best possible talent who share our vision.
"Martin's experience, knowledge and expertise will be invaluable in driving the group forward across a range of sectors."
"Martin has the right combination of wide-ranging delivery experience and a dedication to quality, and health and safety.