You also hear it in Scouse. Rather weirdly Wikipedia describes this as lenition, not pre-aspiration. I suppose it could be somewhere between the two as they are quite similar. You writing muXk does seem to look like something between lenition and pre-aspiration.
When you say 'ch in German ich' I assume you mean 'ch in German 'ach' as both languages have the same broad/slender contrast for ch.
Theory says it is better to learn it by sound first, and the spelling is complicated. Some might consider it cheating, but I use a spell-checker and I consider it helps my learning rather than hinders it. I have found that my browser and my smartphone (when using the website) can be set to Gaelic and do the spell-checking, but I do not know if it is available on all systems or in the app.
When doing Welsh on my phone I actually have to use the spell-checker, as if it is not set I cannot type ŵ or ŷ.
For some odd reason, I can set my phone to check in Welsh and English, thus avoiding switching, but I cannot do this for Gaelic.
I have never heard the word kirk used in Gaelic, and if it were I would expect it to be more like circ (the genitive of cearc with the e left off as is quite common). When the vowel is broad (cearc) there is a quite distinct y-sound in the ce. But with a slender vowel, there is no need for a distinct y-sound. This means that kirk would actually be quite similar in Gaelic and Scots.
The Gaelic translation of kirk is eaglais, and this is the word that would normally be used.
The Kirk is often used to mean 'The Church of Scotland'. This terminology, effectively saying that one particular church is the church, implies that this church is the dominant one. But it isn't amongst Gaels. Lots of Gaels are members of the Free Church (an Eaglais Shaor) and various other related groups, regarding the Church of Scotland as theologically wrong. So they would just say Eaglais na h-Alba or An Eaglais Stéidhichte (The Established Church, since it is the established church in Scotland) on the rare occasions they wished to refer to it.
Note that in this context, 'free' and 'established' are effectively antonyms. Gaels have never been too keen on doing what they were told by the people in Edinburgh.
I only asked because I thought 'cearc' would sound like 'kirk' so I checked it out on the 'Learngaelic.scot dictionary' It appeared to allow 'Kirk' and 'pronounced' it like 'cearc'. In place names 'kirk' seemed to become 'cille' I guess Kilmarnock, Kilmarten...would be the Anglicised versions. Thanks for the background info, it all helps cement the language in my head; Greenock has become a 'sunny hill' to me, since starting this (changed days!)
Yes, cille (Kil- in anglicized place names), probably related to a monk's cell, referred to a Celtic church (more like a tiny monastery than a modern church). The second part of the name is usually the saint the church was dedicated to. The old Celtic Church is not directly related to any modern church. It was effectively abolished (in theory) by the Synod of Whitby, 664, although I do not know if it survived in practice after that.
silmeth has just posted this link on another question, that is relevant and interesting.