'thar' is a preposition meaning 'over, beyond'. 'lear' is an older word for 'ocean, sea'. Thus 'overseas'. Given Ireland is an islands, 'overseas' and 'abroad' basically mean the same thing.
In Irish mythology, 'lear' was a personification/god of the sea. Contrary to appearances, he has nothing to do with 'King Lear'.
Interesting. There are a lot of Irish words for the sea, appropriately enough, knowing how important the sea is.
Muir, Farraige, Lear...