Yes, the Ulster tartan is old. It's strange that people often only make mention of the Ulster-Scots people and the Plantation when referring to it. Most of these people were Lowlanders and would not have worn tartan, much less kilts at that time. It should be remembered that there was a Scottish presence in Ulster long before the Plantation of the 17th Century: the gallowglass. Many were established families by the time the Lowlanders arrived with the Plantation.
Remember, though, that the original material of the Ulster tartan was found as trews, not as a kilt.* The material was dated to between 1590-1650, right around the time of the formation of the Plantation. Yes, there were Highland Scots in Ulster -- the galloglass -- but you cannot discount a Lowlander being the original either. We'll never know the owners' geographic origin or political sympathies for sure.

Tartan was used as a symbol of Jacobite sympathies in the Lowlands, and not every Lowlander was a "true blue whig", to paraphrase David Balfour. So again, you cannot say that no Lowlander ever wore tartan.

Also, you neglected to mention that the saffron kilt was adopted by pipers of some of the Irish regiments right before and during the First World War. No doubt that this adoption was based on the uniforms of the Highland Regiments of the British Army.

Regards,

Todd

*And yes, Highlanders did wear trews, especially the gentry, especially for riding.