Ulster back-row forward Roger Wilson and London Irish centre Nils Mordt have both agreed terms with the Saints.
Wilson was capped by Ireland against Japan in 2005, while Mordt has featured for the England Saxons side.
Wilson said: "Northampton are an ambitious club. They are on their way up and I want to be a part of that.
"I want to improve as a player and I have always wanted to play in England.
"It is a physical league and the quality has risen dramatically. I hope I can fit in to how the game is played there.
"I have been at Ulster so long, it will be different. But it's also a positive move for me."
Northampton, relegated last season, are currently running away with National League One.
They have won all 19 of their games and currently hold a 10-point lead over closest challengers Exeter.