The 31 year old, who played the first of his 37 tests in 2013, is out of contract after being released by the Bristol Bears in May.
Lam, who had been at the club since 2014, captained the side in the early part of the 2016-17 English Premiership campaign but was one of 11 players to be shown the door by his coach and cousin Pat Lam, including fellow Samoa international Tusi Pisi.
He received offers from a couple of clubs in France but said he turned them down in order to play for Samoa.
"It was a no-brainer for me.
"There was a couple of contracts in France that I could have taken but they had the condition that I had to turn down the World Cup and (the chance to) play for Samoa - in the future as well - but I just couldn't do that, seeing I still had too much to offer in the jersey and it just didn't cross my mind," Lam said.
"It was a bit of a risk coming, especially for my family, but (my) missus and family are all behind with by decision."
Samoa coach Steve Jackson previously said a number of players made themselves unavailable for the World Cup because of pressure from their club teams.
Jack Lam admitted it was a tough situation.
"I chose not to take that route or that path," he said.
"Other guys obviously have families as well and they might have seen that as a risk coming here to the World Cup.
"But, hopefully in the future it does change where we won't have these kind of problems where we have to (not) pick certain players purely because they weren't available."
Assistant coach Alistair Rogers said while they may be undermanned it had only brought the team closer together.
"It has been a challenge selecting the squad as Jack says but the great thing about it was we actually knew that we had a group that really wanted to be here, that sacrificed a lot to be here.
"You can build a good foundation with that and we believe we have. I think it has to change really for the future of Pacific Island teams - we'll see where that goes and where that leads to."
Tonight's clash will only be the seventh meeting between Samoa and Ireland, and the first at a Rugby World Cup.
Samoa's solitary success came in 1996, while Jack Lam, Alapati Leiua, Logovi'i Mulipola, Teofilo Paulo and Tusi Pisi were all involved when the teams last played in 2013.
Alistair Rogers said these kind of matches don't come around too often.
"Any time we play a team - I hate to use the words tier one and tier two but a tier one team then it's a great opportunity for us.
"The only way you learn is really by testing yourself against the best and that's what we'd like to do as a team. So, hopefully in the future we get more opportunities to be able to play tier one nations and that will help us to grow," Rogers said.