Orient capture Mulryne
Mulryne, who began his career at Manchester United, was released by Cardiff City six months ago and has spent the past two weeks on loan at Brisbane Road.
The Belfast-born player, who has 25 international caps, did enough to impress Os boss Martin Ling.
"Phil has been to the top and is looking to rebuild after a period out of the game," said Ling.
"The talent is certainly there and now it's up to Phil to show us what he can do between now and the end of the season."