They sound all very beautiful and celtic culture is interesting. But some friends, irish and scottish learners, say that these languages are much more difficult than welsh, and even chinese and japanese are easier in their opinion.
I don't have any experience with other celtic languages, other than welsh. And yes, welsh is challenging, mostly because of mutations. Are there mutations in scottish and irish too? Or there is something even more difficult in the grammar, and/or pronunciation?
I would probably agree that Irish and Scottish Gaelic are slightly more difficult than Welsh, in some respects.
Pronunciation: despite some notoriously difficult sounds in Welsh (especially LL, extremely rare amongst the world's languages: if you speak Xhosa or Navajo it won't be a problem), the pronunciation of Irish and Sc. Gaelic is much more complex: Irish has about twice as many different phonemes as Italian, since each consonant has two different forms, velarised and palatalised (if you know Polish or Russian, these are not such a challenge).
Grammar: certain features of Irish are more difficult, such as the genitive case, which Welsh lacks. But Welsh makes up for this with more verbal inflections and a more complicated answering system (i.e. how to say "yes" and "no"). The grammar of Sc. Gaelic is slightly simpler than Irish but the difference isn't that great and the two are mutually comprehensible to a large degree (especially the Ulster dialect of Irish). Overall, the grammar is fairly similar between the different Celtic languages.
Idiom, or how you say things: I generally have the impression that Welsh is slightly closer to Standard Average European, while Irish and Scottish Gaelic are a bit more "exotic".
Mutations: these are a feature of all the Celtic languages. Welsh has three different mutations: soft, nasal and affricate (though the last two are very restricted in use); Irish has two: lenition and eclipsis; Scottish Gaelic only has one: lenition. Once you get the hang of mutating, it'll come naturally.
Learning resources: Irish probably wins in terms of books and online materials, but Welsh beats them all for numbers of native speakers.
Lastly, don't worry too much about how difficult or easy they are. If you're interested, have a go at learning them (I would heartily recommend all of them), and try to see any difficulties as challenges rather than obstacles; and if you have any problems, just ask.
While I have never learned gaelic I can tell you that it shares things in common with Welsh. The grammar is the same. Ex. Word order is verb subject object. All celtic languages share this. There are mutations but the difference lies in the overall pronunciation. Irish and Scottish used to be the same language and so there's a range of pronunciation that meets where the two countries do also. If you need help tutoring with Welsh just contact me.