I haven't found a noun meaning "mindfulness", besides the modern term I posted above, but the following link shows some of the possibilities of the adjective "mindful".
http://www.dil.ie/advanced_search?q=min ... ch_resultsFor "a grieving person" or some such possible word, I couldn't find anything but you could consider what is posted at the bottom of this message from this link:
http://www.dil.ie/search?q=griefI bolded the phrase "grief-stricken man" which is the closest thing I can find to what you are looking for.
The Old Irish for that is in the genitive so you'd want to put it in the nominative. It appears the word for "man" in the nominative is "fir" and then "grief-stricken" is aithmélach > fir aithmélach - "a grief-stricken man". There are variations in spelling of the adjective as you can see below while I've seen "fir" spelled "fer" and also the word "duini" or "duine" used which means "man" in the general sense of "person" or "people".
Sorry I can't help you more. There are some people who post here sometimes that know a lot more thN I do about the older language. I don't know their posting habits unfortunately. But you can search around yourself and maybe find something I missed. Good luck with that.
Cheers,
Tim
aithmélach
Cite this: eDIL s.v. aithmélach or dil.ie/2746
Forms: aithméilech
adj o, ā (aithméla). Also aithméilech.
(a) a ngēirsgiūrsadh go himnáir aithmēlach seachnōin na srāide ignominiously , Fl. Earls 64.10 .
(b) remorseful, contrite, grief-stricken: as aithrech aithmelach lem a ndernus I am sorry for what I have done, BNnÉ 237 § 162 . ba haithmhēlach imnāir lé a ndorōine, Fl. Earls 34.19 . aithmhéileach dhúin a ndéanmais, TD 3.6 . caoi ḟir aithmhéalaigh the cry of a
grief-stricken man, Aithd. D. 78.18 . Adv. docuaidh dha íarraid co himnideach aithmhelach `in anxiety and contrition' (of a priest who had lost the consecrated Host), Ériu ii 82.23 . rugatar as an adaig sin go haitmélach, gan ol gan aibhnes despondently, Celt. Rev. iv 10.1 .