Hmmm, hard to say. I think both Irish/Gaelic and Scottish sound kind of cool and have some really nice accents when a native speaker switches to english.
That said, I think it's more a matter of the speaker than the languages themselves, and region probably plays a big role in it as well. I've run into a LOT of people with a LOT of differant accents over the years, and I'd I've heard pretty much any major language spoken that you can think of. I don't think any language is inherantly pretty sounding all on it's own.
But then again, as I said a lot of this has to do with region and other things. It's sort of like how you take an english speaker from a really poor area using a lot of slang or a sub-dialect like ebonics, and even if your a native speaker it can really hurt your ears or make you go "huh, what was that guy saying?". It's like the differance between thinking a southern drawl is cool/sexy, and then hearing someone staggering around an Alabama trailer park.
Most languages have a lot of regional accents and sub-dialects (even if most people can understand them). My ear for some languages is better than others, but there can be a substantial differance in say european/english accents based on where in the UK someone is from. Also when I'm talking about lower class dialects with heavy slang attached you see the equivilent over there in things like the "Chav" subculture, or heck even the stereotypical "hooligans" that they imitate for the voices of say Warhammer 40k orks.