Back to the meaning of 'to shine', it seems out of place. Nonetheless, Mallory and Adams in the Encyclopaedia of Indo-European Culture (1997), page 514, tries to assure the reader that the 'shine' root is "widespread in IE". Yet a closer inspection of the evidence shows something suspect:
- Not one cognate shows the unreduplicated verb root and meaning directly.
(ie. All are obscure derivatives.)
- Many things are left unexplained.
(eg. How is Sanskrit marīci- 'shining mote' to be analysed morphologically? Doesn't the unusual pattern of reduplication in Greek μαρμαίρω [marmáirō] deserve explanation?)
- Taking each branch separately, the verb is weakly evidenced.
- Some of the cognates can equally be interpreted using a different sense of *mer-.
- Added all up, there are an unusual number of odd problems demanding special pleading.
 Puhvel, Hittite Etymological Dictionary, vol.6 - Words beginning with M (1984), p.150 (see link).