What all that means is this:
1. The word
alla doesn’t really mean anything on its own. It’s a kind of
cranberry morpheme (though it’s a full word, not a morpheme) that only has meaning when attached to another word, which it then narrows down.
2.
Damhán means ‘little ox’ transparently: it’s a diminutive of
damh ‘ox’. It also has an entomological meaning that somehow deals with spiders, though it doesn’t really mean any specific insect on its own.
3.
Ruán is a kind of fish that, for whatever reason, can also, when coupled with
alla, be used to refer to a spider.
A possible (though hypothetical—I don’t know the actual history of these words, so I’m guessin) scenario of how it could have evolved:
It’s quite likely that
alla is an old word that means ‘spider’, but which has lost its original meaning. At some point, very likely compound names were formed: an ‘ox spider’, a ‘water spider’, and a ‘rudd spider’. At some point,
alla ceased to simply mean ‘spider’, and the most common of these words, the ‘ox spider’, came to be used generically for all spiders, so the ‘ox’ bit came to mean just ‘spider’. And voilà, you have the silly hodgepodge of words and forms that are used today.
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OR - perhaps it backs it up in a slightly altered way: alla meant "spikey" and so the damh alla is "a spikey ox" (think of the antlers). And a damhán alla is "a little stag". But damh also means "a strong person" so a damhán is "a strong little bugger" and damhán alla is "a spikey strong little bugger". And a ruán is "a little red thing" so ruán alla is a "a spikey little red bugger". None of which, or course, tells us why buggery has nothing to do with insects or your oxter has nothing to do with castrated bulls ( 
We have very plain ones in Ireland Breandán 
I thought you would have noticed. 
