Irish Eclipsis/Lenition and Welsh Mutations.
Hi, I'm from Wales and Welsh is one of my two native languages. I've noticed that in Irish when something is mutated the usual letter is kept and the new letter is added in front of it. In Welsh we replace the normal letter with a new one e.g "merch" (girl) becomes "y ferch" (the girl). I was wondering if there is any historical reason that the original letter is kept in Irish? Diolch. (Go raibh maith agat)
Originally (in Old Irish, as well as in early Middle Irish), mutations were generally not shown in the spelling (the same thing is true for Old Welsh, although it had more or less adopted modern conventions for the spelling of mutations by the Middle Welsh period). The exceptions were, for example, lenition of t to th (which was pronounced similarly to th in English thing, or Welsh arth back then).
During the Middle Irish period it became increasingly common to show lenition by adding a dot over the affected letter. This spread out of the usage of adding dots over letters to show spelling mistakes, which should not be read, much like we might scratch out a mistake by crossing it. Since the lenited counterpart of f is not pronounced, it was quite simple to extend the system to show lenition of f; and once this was established the dot became common to show lenition of all consonants.
However, the model of adding a -h to mark lenition, as mentioned found e.g. in t to th, was also extended and eventually, during the late Modern Irish period, became the established norm for marking lenition.
The retention of the original consonant (in writing) in the case of eclipsis is another late reform added to improve the readability of the language, although there are scholars who believe that in early Old Irish eclipsis of e.g. b to mb was also actually pronounced as /mb/, rather than Modern /m/.
I hope this helps!