A look at the Celtic languages from the point of view of how hard they are to learn for an English speaker.
The verbal system in Old Irish is one of most complicated of all of the classical languages.
Irish students take Irish for 13 years, and some take French for five years. These students typically know French better than Irish. There are inflections for the inflections of the inflections, a convoluted aspiration system, and no words for yes or no. The system of initial consonant mutation is quite baffling. Noun declension is mystifying. Irish has irregular nouns, but there are not many of them –
the woman – an bhean
the women – na mná
and there are only about 10 irregular verbs. There are dozens of different declension types for verbs. The various phonological gradations, lenitions and eclipses are not particularly regular. There are “slender” and “broad” variants of many of the consonants, and it is hard to tell the difference between them when you hear them. Many learners find the slender/broad consonants the hardest part of Irish.
Irish and Old Irish get 4.5 ratings, extremely difficult.
Both Scots Gaelic and Irish Gaelic are written with non-phonetic spelling that is even more convoluted and irrational than English. This archaic spelling is in drastic need of revision, and it makes learners not want to learn the language. For instance, in Scots Gaelic, the word for taxi is tacsaidh, although the word is pronounced the same as the English word. There are simply too many unnecessary letters for too few sounds. Of the two, Scots Gaelic is harder due to many silent consonants.
Irish actually has rules for its convoluted spelling, and once you figure out the rules, it is fairly straightforward as it is quite regular and it is actually rational in its own way. In addition, Irish recently underwent a spelling reform. The Irish spelling system does make sense in an odd way, as it marks things such as palatalization and velarization.
Scottish Gaelic and Manx have gone a long time with no spelling reforms.
Scottish Gaelic gets a 4.5 ratings, extremely difficult.
Manx is probably the worst Gaelic language of all in terms of its spelling since it has Gaelic spelling yet uses an orthography based on English which results in a crazy mix.
Manx gets a 4.5 ratings, extremely difficult.
Welsh is also very hard to learn, although Welsh has no case compared to Irish’s two cases. And Welsh has a mere five irregular verbs. The Byrthonic languages like Welsh and Breton are easier to learn than Gaelic languages like Irish and Scots Gaelic. One reason is because Welsh is written with a logical, phonetic alphabet. Welsh is also simpler grammar-wise, but things like initial consonant mutations can still seem pretty confusing and are difficult for the non-Celtic speaker to master and understand. Verbal declension is irregular.
caraf I love carwn we love cerais I loved carasom we loved
The problem above is that one cannot find any morpheme that means 1st person, 3rd person, or past tense in the examples. Even car- itself can change, and in connected speech often surfaces as gar-/ger-. And carwn can mean I was loving (imperfect) in addition to we love. There are no rules here, and you simply have to memorize the different forms.
Welsh and Breton get a 4 rating, very hard to learn.
Breton is about in the same ballpark as Welsh. It has a flexible grammar, a logical orthography and only four irregular verbs.
On the other hand, there are very few language learning materials, and most of those available are only written in French.
Breton gets a 4 rating, very hard to learn.