Scunner is Middle English, derived from 'skurnen' meaning to flinch. In current North Yorkshire and Scots usage, it implies disgust, revulsion as a verb and a disgusting person as a noun. I believe it to be related to 'scon' and 'scunner' (v) in Cumbrian speech, meaning to shock or stun. (When I was a youngster, to 'Scon' someone was to hit him on the head, typicaly with a stone... "He sconned him wi' a gurt clemmie".)
From that, I'd hypothesise that the song implies that white horse (either the Hanoverian emblem or, more likley, the older one of the Saxons)has fouled the Irish landscape, and exiled the native people.

JWB- I have your CD, my thanks and congratulations! Excellent!