O’Neill said: “I thought Jim's other commitments in Northern Ireland and the UK would prevent him helping me at this crucial stage of the season. I was delighted when he said he would assist me until I find the right candidate for the job on a permanent basis.
“He brings not only a wealth of knowledge and experience of the game at the highest level, but he is someone I trust implicitly, and that was a key factor in my decision.”
Magilton – a former Northern Ireland captain – has managed Ipswich Town and QPR in the English Championship.
O’Neill will continue to consider candidates for assistant manager at the club.