@k4523
Rua quite popular in Ireland
Rua was ranked 324th in baby boy names in Ireland last year, there were also a few named Ruadh. However, this is a recent use, as Rua was commonly used as a descriptor in the past, rather than a name as such. It means red (more specifically rust-coloured) and is the word used to describe red hair colour. So Eoin Rua is red-haired Eoin, while Eoin Fionn is fair-haired Eoin, for example. This way of distinguishing people has mostly fallen out of use now though.
Rue makes me think of regret as pp have said, OP.