Dia Dhuit, fellow kilties

There are many interesting points to address in these replies.

Most historians agree that the Highland Scots wore clothing much like the Irish (Saffron Leine and brat, etc.) until the 15th Century or so. Thus in a sense the kilt is loosely derived from Irish clothing as the Scots began to belt their plaid instead of wearing it loose over their clothing. It seems somewhat fitting and complimentary that modern Irish "traditional" dress should mimic Scottish Highland traditional dress. The Gaelic circle comes back around.
The Kilt was not even thought of as quintessentially Scottish until Sir Walter Scott. Previously it was regarded with some degree of scorn by the Lowlanders. It was only with the Highland revival of the 19th century that the kilt became a pan-Scottish garment.
Like the Irish saffron kilt (mistakenly derived from the saffron leine) the modern Scottish kilt is also based on a certain amount of historical fallacy. Most "family" tartans were simply designed by textile manufacturers of the 18th century and were assigned names instead of numbers. It could be argued, I suppose, that many of these samples were gathered from existing sources. Maybe so, but they would likely have been regional tartans at best.
The saffron kilt was believed to have been adopted in the late 19th Century as a sign of Irish nationalism. The Highland kilt was adopted in the early part of the 19th Century by many Highlanders and Lowlanders alike as a sign of Scottish nationalism (or cultural identity, if you prefer). This puts the Irish tradition as only a handful of decades younger than the modern Scottish tradition.
As the Highland Scots are partly descended from the Irish, they kept a close cultural and political connection with each other for many centuries. Gael Albanach (Scottish Gael = Highlander), Gael Eireannach (Irish Gael), and even Gael Mheiriceanach (Irish American) are all Irish words which emphasize this connection. Unifying the traditional dresses could be seen by some as bridging the gap that was sundered long ago.
I'm not saying the kilt isn't Scottish, far from it! But it should be noted that Historically the Highlanders indentified with the Irish far more than with the Lowlanders. Companies, however, should not concoct false histories to loosen the claim from Scotland, nor should they validate Scottish fables such as tartan ogham and other nonsense!
In all, the Scots should be proud that they are really the only Celtic people to have retained their traditional form of Celtic dress into modern times. The other Celtic cultures look to them for inspiration. It can be used as a powerful symbol for Celtic pride everywhere but should never be used to further anti-English sentiment. One may be angered by the history of the past and by certain political policies, but one should never downtrod another ethnic group. The English are a fine people with a fine culture.

Éire agus Albann go brath!

is mise le meas,

Seán Liosliath Ó hAirt