Can you tell where this text is from? Searching this text in modernized spelling leads me nowhere, except some Russian sites.
This can also be found an an old Estonian paper encyclopaedia in late eighties as an example of Irish letters.

http://tapemark.narod.ru/opred/125.png

Well, not every book in Irish has been transcribed online. From the text, the person has clearly climbed to the top of a hill in Co. Kerry, and so that gives a clue of some kind.

Judging by the description of the view, the writer would have been standing on a mountain peak in the western half of West Kerry, possibly on Mount Brandon. But I have no idea who is writing it.

Sionnainne is a spelling mistake - it should be Sionainne.

Ceart go leor, GRMA. Ós rud é go raibh sé clóbhuailte i seanlitreacha agus mar shampla de i ciclipéid, shíl mé gurbh théacs clasaiceach é agus aithne air ag duine éigin a a laghad. Admhaín go raibh dóchas beag ann, ach gur fiú iarracht a bhaint as.

Another noticeable thing is "feiscint" instead of "feisgint", as at one point sg- was thought to be the correct spelling. When Dinneen's dictionary was commissioned, the majority of experts consulted agreed it should be sg- and Dinneen was told to use sg-, but he disagreed and pleased himself and used sc- and thus presented the Irish Texts Society (I think it was them) with a fait accompli that has stuck. This may mean your excerpt was written between the time Dinneen's dictionary came out and the time that Ó Dónaill's dictionary came out???

Seems plausible at first sight.
However, Ó Dónaill came out in 1977, but according to Wikipedia, gaelic types were used until mid-20th century, perhaps no further than 1950s or early 1960s. And it seems to me that one doesn't need to consult dictionaries to use 'sc' instead of 'sg'. That could independently occur to anyone not too entrenched in traditions, as it is the actual way they are pronounced.

Is it? Older speakers in the Munster Gaeltacht have been known to say - I've been told by them - that the pronunciation is sg-.

I think it is clearly /sk/, but unlike k elsewhere, it is not aspirated (with a puff of breath), and so may seem to non-linguists to be like a g. But even Brian Ó Cuív in the Irish of West Muskerry (an incredible source otherwise) stated the pronunciation was with g. I think he must have meant, not a voiced g, but a non-aspirated k. So you see the confusion.

I regard sc as a spelling mistake tout court. Because in any language there are things in the orthography that don't correspond to the pronunciation as such. Like "i mBaile Átha Cliath", where the pronunciation is "a mBleá Cliath", where the "i" is more like an "a". But as this spelling mistake is insisted on by everyone, I don't think the sg spelling is available nowadays, and so when I republished Niamh by Peadar Ua Laoghaire, I used sc.

I think his use of 'feiscint' indicates that he's from Munster. Connacht and Ulster use 'feiceáil', I think. There are a couple of other indicators in the text that the writer is from Munster.

This touches upon a topic that is also discussed in this interesting video: https://www.youtube.com/watch?v=U37hX8NPgjQ, (non-aspirated vs. aspirated consonants). A version of that video for the Irish language would be nice.