I’ve no idea
why but seems it’s a pretty old thing. DIL:
In later lang. [ie. in Early Modern Irish, maybe MidIr. too] the prot[otonic, ie. dependent] stem dén- is almost entirely replaced by dern- throughout the subjunctive (IGT Verbs §1).
IGT Verbs §1 lists:
Quote:
(…) gé chuin do-neis a .s. ó indscni thú, gé chuin do-né on dá innscne ele : gé chuin ná derna .s. a .nd. araen. (…) gé chuin do-neam : gé chuin ná dernam, gé chuin nach dernam a .s. (…) gé chuin do-neithí, gé chuin na derntaí a .s. (…) gé chuin do-net, gé chuin ná dernad a .s. (…) gé chuin do-nethear : gé chuin ná derntar a .s.
It has indep.
do-né vs dep.
ná dearna.
McKenna in
Aithdioghluim Dána lists these forms:
Léamh glossary, s.v.
do-ním wrote:
Subj. do-near, -dearnar; do-néis, -dearna; do-né, -dearna: do-neam, -dearnam; do-neithe, -dearntaoi; do-nead, -dearnad; do-neithir, -dearntar;
So seems like the dep. form was pretty much thoroughly
-dearna by Classical Gaelic times – but also for regular verbs the pres. subjunctive was mostly identical with present tense, so that was a push for forms without the -r-, I guess?