Scottish Gaelic + Irish
Scottish Gaelic is so much like the Irish language, a lot of the sentences are identical. I've only started Scottish Gaelic but I'm finding it a breeze as an Irish speaker. Manx (from the Isle of Man, or Ellan Vannin) is also extremely similar. Manx is even easier to understand reading because it is written phonetically. Just if anyone wants a brain challenge, try one of these languages as well as Scottish Gaelic to see how you get on!
There is a phonetically spelled Celtic language?! Why isn't Manx on Duolingo yet? XD
But yes, the similar grammar with the different teaching approaches is helping me with both languages. I'm just reaching the point where I'm starting to mix up some words (I wrote "fiche" instead of "fichead" twice yesterday... XD).
The Conversational Manx blog has Manx phrases and sentences written out in Manx spelling and Gaelic/Irish spelling underneath (the words written as they are written in Irish or Gaelic when they exist in those languages, and sometimes as they would be phonetically in Irish spelling, so you plural is written siú – instead of sibh).
Helpful if you want to compare Manx to the other two languages.
Manx is even easier to understand reading because it is written phonetically.
Funny, I find Manx very hard to read (but then, as a foreign learner, I am not that used to Welsh-and-Englishy kind of spelling, nor to spoken Irish while I am used to the more etymological Irish and Gaelic spelling).
The Akerbeltz article on Manx spelling summarizes nicely the points in which I don’t like the spelling either. (But then, that’s Manx tradition, so nothing for me to rant about if the Manx are happy about it – I just personally don’t find it that easy and nice ;-)).
I believe the Akerbeltz article makes a pretty strong point that it is nowhere close to being intuitively phonetic even for English speakers: th meaning /t/, (e)y meaning /ə/ (and not /i/, /ai/, or /ei/ as English speaker might expect), ch for /x/ while /tʃ/ (the English ch) is written as çh, etc.