For what it’s worth, bilingual road signs (when the second language is Gaeilge) are more costly to create. There must be tens of thousands of signs across Ireland that declare Rampaí, which means signs are bigger and need more ink.
More importantly, the increased levels of text involved in bilingualism decreases the legibility of those signs, and therefore decreases their ability to deliver information instantaneously. As such they could be perceived as a distraction to drivers.... and therefore make driving more dangerous.
The opposite might apply in somewhere like Croatia, whereby their influx of tourists undoubtedly struggle with the unusual characters in their lanaguage, and providing an English language version of the message helps prevent tourists from slowing to a stop at junctions and roundabouts.
But in Ireland, where every single person who speaks Irish also speaks English, and tourists have no understanding of Irish, I do believe it’s an unnecessary distraction.