The two conflicts weren't fundamentally that different. In both cases, you had insurgent forces operating waging an irregular campaign on behalf of a provisional government engaged in a dispute with the British government over sovereignty of a given territory. In both conflicts, Republican and Loyalists paramilitaries used a combination of irregular and terrorist tactics, and at the height of the Troubles- the late '60s through early '80s- irregular warfare rather than terrorism was predominate within the Six Counties themselves. The difference, in terms of how we categorise it, really comes down to the fact that the earlier war produced a definite geopolitical reorganisation, while the later would just produced an awkward stalemate.