The etymology of potato is quite interesting. The English word potato originates from the original Tainoan word batata trough Spanish patata. Now both of these words actually mean sweet potato, but in English it has had to accept the sweet part for itself.
In other European languages potatoes were originally known largely as either earth apples or earth pears. For example it's still pomme de terre (apple of the earth) in French.
When the potato finally arrived to Finland during the 18th century we adopted the Swedish jordpäron (earth pear) to name it, or just päron and later it just evolved from that into peruna as we know it today. Sometimes we slso call them pottu or potaatti which are rooted closer to the original term patata and not the english potato even if they sound more similar. The sweet potato in Finnish is actually bataatti.
It is also pure incident that there is peru in peruna. One could think it could come from something like perun nauris (peruvian turnip) as potatoes did replace the earlier widely used turnip in Finnish diet soon after it's arrival and potatoes originate from Peru. At least it works as s nice mnemonic for us.