I don't know when Irish died out in Limerick but I imagine it was fairly early. Certainly I'd expect well before 1885, which is when Dick Maitland first heard "Liverpool".

I'd be almost certain that "Limerick" isn't a translation from the Irish. In fact very few Gaelic songs made it into the Anglo-Irish folk tradition. Plus there aren't all that many emigration songs in Irish anyway. In fact the vast bulk of emigration songs postdate the famine of the mid 1840s and are in English.

It's likely therfore that Limerick was written some time from the mid nineteenth century on. The reason why I think Limerick-Liverpool is more likely than Liverpool-Limerick is simply tht it would reflect the pattern of emigration. At that time, emigration was very much a one way ticket and you didn't get much in the way of returnees. Hence a song written in Limerick had more chance of being turned up in Liverpool than a song written in Liverpool would have had of finding its way to Limerick.

On the other hand, the way that sailors kicked about the world, you would never know for certain.