Actually, your intuition on this is right. "a" is a (late ?) alternate for "i", so I expect that he went "into" Ulster.
'Luidh fecht ann a crich nUlad'
I don't understand the use of the preposition 'a' (out of, from)
It looks like 'one time he went out from the territory of Ulster'
or 'out from
the border of Ulster'. It doesn't translate that he 'went to Ulster',
would make more sense in the context of going to kill an Ulsterman.
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