The bombs don't ask whether a potential victim is British before going boom; this stuff has to be hurting America even more than Britain. What beef does the IRA have with America? Why do they want to tempt the eagle's reprisal?
They're terrorists. You might as well ask what beef Al Qaeda has with the US. Does it matter? If they have a 'legitimate' beef does that make it ok to bomb people? The IRA have always been on the same level as Al Qaeda. They're all murderous bastards who need to be destroyed. It should surprise no one that they are working on the same side.
You assume these guys are following orders. There's no reason to believe that. For some folks, the struggle becomes a cause in itself, and when their war is over, rather than lay down arms, they find another war for a paying client. They go freelance.
During the Cold War, the USSR would back pretty much anyone who attacked the West. IRA terrorists received money, arms and training from the Soviets, funneled through the PLO and Libya.
When the IRA officially decommissioned, some of their most accomplished bomb-builders were suddenly out of a job. A whole lot of other awful people were out of work at the KGB and Stasi after 1991. They had a well-established network, time on their hands and skills that some folks would pay a lot of money for. Pull back those sheets, and you'll find mighty strange and nasty bedfellows.
Colombian officials blamed IRA bomb-makers for training FARC narcoterrorists, but couldn't make the case and eventually had to let them go. The Basque terrorists in ETA have adopted what appear to be IRA techniques and technology, though the links have been impossible to prove so far. So it's not much of a stretch to believe that the Iraqi insurgents, and their financial backers, chased the expertise and the experts followed the money.
i remember hearing once that more IRA members were killed by rival IRA factions than by British soldiers.(this was in reference to the irish terrorist campaign to achieve independence, which was followed by a civil war between 2 ira factions.)
my impression is that the current crop of terrorists in Ireland are sociopathic gangsters.