Unlikely, it's more likely to be a home grown Northern Irish group.
That does seem to be most probable, given the circumstances as described. Still, there are so many possible candidates these days, it's best not to rush to judgement on the origin.
One of the more endearing features of terrorists is that they are usually dying (literally in the case of jihadists) to claim credit for their atrocities, so much so that there are often multiple groups claiming credit for the publicity value. This makes it
so much easier to decide who to hunt down and kill.
If they just wanted to kill people without getting notoriety for it, they would all be trying to make it look like some other, possibly-rival group did it, but no, they don't think that way.