They are not. However, Russian forests are very comparable to Lithuanian forests... them being Russian forests in the first place and only "borrowed" by Lithuanians for a century or three. If Mongols can massacre tens of thousands of Russians in Vladimir-Suzdal region, they can do so to Irish in Lithuania. Historic? No. Plausible, yes.
Aye, people seem to frequently underestimate the Mongols ability to adapt to terrain. While they may not have been as effective in certain terrains, they still most likely would've made the best of it.
And personally, I think the only obnoxiousness of the OP's screenshot is that the Mongols can walk around with 90k guys in a single county and have no problem. Otherwise, the Mongols are absolutely fine right now. In my Nubia game I've personally held the line against the Ilkhanate since damn near their arrival, slowly pushing them out of the holy land. The Golden Horde started off strong as they got to ignore the Cumans (there was an ever so thin group of Cuman breakaways in northern Perm they used to essentially walk along the sidelines of the game world until they hit the Russians), and they'd go on to take all of Rus, most of Lithuania, and all of Scandinavia.... then the Crusades started. Three Crusades, one for Finland, Norway, and Sweden respectively, and only the one for Finland failed. After that, the Golden Horde has been regularly erupting into civil wars.
Similar for the Ilkhanate. They took Khiva (well, the southern half at least... some is still by Cumania), Persia, Mesopotamia, Arabia, Syria, and a bit of Jerusalem and Armenia. Then, however, I started regularly fighting to push them out of Jerusalem (most of which I held already when they arrived) and Syria. It was mostly status quo, with me scrapping against their invasions and trying to sneak in a holy war here and there when I could... but finally, they attempted to invade my friends in the independent kingdom of Armenia. I offered to help, and we crushed em', killing two Khans in the process. The other son of the original (from that war) Khan then tried to retaliate with an invasion against me... which also failed. With their levies completely depleted, he apparently accepted the demands of a faction and stepped down for yet another new Khan... and its been a mess to my east since then. I don't think since the end of the 5th Mongol Invasion of Syria that there's been a moment where the Ilkhanate hasn't had a civil war going on.