As Matt and various others have pointed out, the Irish county tartans are a purely Scottish invention. The colours you mention are probable taken from the Cork (city and county) crest, which is two towers (red) with an orangey-yellow background. The towers are usually depicted on two hills (green) and there is a ship sailing between the towers (the sea is blue). Hence the 4 colours you mention for the Edgar tartan. However, the land and sea are not always coloured, which could give you just red, yellow and orange. I haven't seen that tartan though.

Now, if you were to ask a Corkman what were the County Cork colours, red and white would be the answer because those are the GAA colours. I doubt very much if the designers of the tartans realised just how important the GAA teams are to each county's identity. The soccer strips have been several colours, though not a lot of orange has appeared....

A degree of ignorance of Ireland, especially the Republic, is displayed by the designer of the tartans through several clues which I won't mention here as it could veer towards the political...