Hi blackcat. Yes, ollamh is obscure but I'd come across it in another croossword some time ago. Unfamiliar vocabulary makes puzzles harder. I seldom finish Mephisto in The Sunday Times without having had to use a word I previously didn't know.
I wouldn't call the Speccie the hardest crossword althought the unclued element offers a particular problem. I just keep plodding along solving the clued words and hope that that gives me enough to make an intelligent guess at the rest. For instance, in 2487, I spotted Night at 37 and then when I'd added 45 I was on the home straight.
Others may have another view but I find, of the puzzles which I regularly see here in the UK, the Listener puzzle is the hardest. It's published in The Times on Saturday and is thus behind a paywall but Tatters, on this site, has pre-publication access to it and posts a link to it here at about teatime on a Friday. Half the time, I can't get my head around the instruction rubric, let alone solve the thing!