Spence was killed off-camera immediately after the team left.
He had just demonstrated that a) he'd lied to a terrorist wing of the IRA and b) he's kind of an idiot. Even if Dierdre was willing to leave him alive (unlikely), Seamus certainly wouldn't be. Dierdre made a show of bribing/threatening him into silence because, if she'd just executed him right there, the rest of the team might think they'd turn into a cost-cutting exercise after the heist and the whole plan would fall apart.
Honestly, I was half-expecting him to show up during the climax, maybe working for the Russians. Still, I think that if that was supposed to be the idea, the film would have shown or at least hinted at it.
- In the Director's commentary, they did shoot a scene where Spence is picked up off the street and killed or some other resolution, but they couldn't make it work due to pacing, so they just let the character drop off.
- On the other hand, it's plausible that Seamus killed Spence as he had been shadowing the team.
