If not an attack, how about something like an arms deal gone bad? Just off the top of my head:
An Irish-American soldier somehow gets it into his head that no one will notice if a couple of mortars go missing. He offers to sell these to the IRA through contacts in Boston. The IRA leadership realizes that a connection like this could be valuable in the future, and orders a small group to go buy the weapons. The terrorists fly into Canada on forged UK passports, and slip over the poorly-guarded border. As insanely risky as it is, they agree to meet their contact on his army base. He promises that he can get them through security as his personal guests. The IRA is extremely worried, but decide to take the chance. They know that their contact would never be able to smuggle out the equipment on his own, and they hope a small group, working quickly, will have a better chance.
Worried about the risk they are taking, the IRA members order their Boston contacts to loan them weapons. Two of the men receive FN High-Power 9mm pistols. The last man, the group leader, is given a Czech Skorpion machine pistol. Finally, on the appointed day, they all load into the American's truck, and head to the base. At the gate, the guard is an old friend of the soldier, as he knew hw would be. The soldier explains that he has friends visiting from "the old country", and he just wants to show them around. The guard waves the truck through the checkpoint.
The soldier parks his truck behind on of the weapons storage buildings on the edge of the camp, where they hope it will be out of sight. The American has the key, and unlocks the door as quitely as possible. The four men begin loading the truck. Suddenly, a walking patrol member spots them. He comes over to investigate, and before the American can try to explain, he draws his weapon. An IRA member, unseen in the interior of the building, shoots him dead. The sound of a gunshot echoes through the night air, and they all know the game is up.
The truck speeds toward the checkpoint, desperately trying to flee the camp. It is clear that they do not intend to stop. US soldiers open fire on the truck, and automatic fire answers from the interior of the truck. Within a minute, the firefight is over. All three terrorists, and their American accomplice, are dead. So too are three US soldiers, with another injured. The IRA has apparently attacked US soldiers, on US soil, during an attempt to steal arms. Even more worrying to many is the idea that "Irish sympathizers" have infiltrated the armed forces, and are willing to sell out their nation. US public opinion towards Ireland hardens overnight...
So, what do you guys think? Hopefully a bit more plausible than an outright bombing attack, anyway.