There is a tendency in re-enactment groups and Ren Faires to lump the Irish and Scottish together and call them “Celts”. This is not a correct term. “Celtic” is a language grouping. It refers to a number of vastly different tribes who were roaming Western Europe around the time of the Romans. Number one, this term includes a lot more people than just the Scottish and Irish. It includes the Welsh, Bretons, Cornish, some people on a northern peninsula in Spain, possibly even some Turks. And these people all had vastly different non-Celtic cultures by the 10th and especially the 16th century. Also although the Irish and Scottish even today can be considered a Celtic people, calling them “Celts” is like calling 20th century English people “Anglo-Saxons”. It’s not wrong, in the strictest sense, but misleading for its lack of temporal accuracy.
There is a word, however, that allows us to lump the Irish and Scottish together under one heading and use a collective term on them. This word is “Gaelic” or “the Gael”. The Irish, Scottish and Manx are descended from the same stock and all speak Gaelic. In the last 200 years, this language has diverged and become three languages, Irish Gaelic, Scots Gaelic and Manx, but the relationship remains. I encourage you to use “Gaelic” when referring to Scots and Irish. You will sound very well read to the people who know the difference.