The actual native Gaelic script is ancient, it occurred way before any of the complexities of modern Irish or Scottish (mutation, broad/slender consonants), and was made up of nothing but lines off a central stem-line (ogham). For the most part, Gaelic is pretty straightforward, at least as much as English and probably moreso. I don't have some of the stranger vowel trigraphs and such down, but I can generally get a good guess despite never studying the language.
I can't imagine someone that's said that did much research. Then again, I have yet to read anything that seems to come up with a decent list, even ignoring the huge cultural bias of what is or isn't difficult. It's all well-known languages that pick out one or two pieces of the language, and even then it seems to focus on the writing and/or one or two particular sounds (which a less-known language invariably is far more difficult). I won't be impressed until I see a list including things like Taa, Koasati, or Georgian, that have a combination of sounds, grammar, sentence structure, as so on radically unlike English. Even for English speakers, languages like Hindi are going to kick someone's ass if they're not prepared for it, and probably deserves a title for hardest language as much as Chinese or Irish or Arabic.I've read that it's one of the hardest languages in the world to learn.
Also, on Welsh ll - it's an easy sound! Many English dialects even make the sound (or one extremely close) in words like click or clear or please (Dumbledore's pronunciation of "clear" in one of the Harry Potter movies stood out as a near-perfect example of it). It's a voiceless el, or an ess made with the air going over the sides of the tongue rather than the top.
On Synonyms:
A lot - but not all - are because of different source languages, but each pair was their own peculiarities. Guard/ward is my favorite examples because they're both Germanic - one through Old English and one loaned into Norman and then back into English. They are not complete synonyms, and each takes on their own specific meaning that the other cannot fill. I'd bet genuine, complete synonyms are nearly non-existent.