Well I happen to have an Irish name, so I’ll have a go.

‘Who cares?’ Has the same ‘who gives a damn?’ meaning on this side of the Atlantic. The sign is to attract attention and make you think. On the one hand, perhaps drivers who are still being careless are displaying a ‘Who cares’ attitude, on the other hand if you answer the question with the inward thought ‘I do’, then you will drive more carefully after seeing the sign; at least until you are out of Westmeath!