Fjelde, 22, recently left Byrne in his native Norway and his move is subject to international clearance.
Scottish forward Loy, 28, was previously loaned to St Mirren by Rangers in 2010.
The Buddies - bottom of the Scottish Championship - visit Dunfermline Athletic on Saturday.
Former Viking Stavanger player Fjelde told St Mirren's website: "It's a big challenge ahead but I'm looking forward to it and hopefully together we can stay up. It's always been my dream to play in the UK."
And ex-Falkirk forward Loy, who has 58 career goals, said: "I've played in this division for two years and did very well in it so I'm not sure it's a case of having anything to prove. I want to score goals and I want to help keep the club up."
St Mirren have signed midfielder Pal Fjelde and striker Rory Loy until the end of the season, with the latter joining on loan from Dundee.