The ancient Greeks coined the word Celt (plural Celtoi) and it referred to the Galátai / Galli.
The Romans then designated everyone to the west of the Rhine “Celts” and everyone to the east of it (and west of the River Vistula) “Germans”.
In the 7th century BCE, Celtic languages were spoken all over Europe. In the map, you can see the area of Transalpine Gaul, north of the Alps, spoke Lepontic. The Gaulish area on the Italian side of the Alps is called Cisalpine Gaul.
The term “Celts” was applied as a catch-all to peoples speaking Goidelic languages (Scottish, Irish, Manx) and peoples speaking Brythonic languages (Welsh, Cornish, Breton). Brythonic languages are also called P-Celtic; Goidelic languages are also called Q-Celtic. The Welsh word for “son of” is “ap”; the Scots Gaelic word is “Mac”.
As you can see from the language family tree, Goidelic and Brythonic are two distinct branches of the language tree.
Each of these cultures is unique and different. They tend to get conflated and romanticized by nineteenth century writers looking for a pan-Celtic culture. They do share many festivals and similar words for things but they are unique.