Technically, it is "It is not but a mouse." which we would say as "It is only a mouse.", more often than not. "There is" is an expression that may be worded differently in Irish and your second "there" is a location that has not been indicated, because this word "ann" is part of an expression "Níl ann ach". We don't know where the mouse is, just that that is all it is.
http://www.teanglann.ie/en/eid/%22there_is%22 Seems to use "tá" , but níl is the negative form, isn't it?
http://www.teanglann.ie/en/eid/%22there%22 I see why you asked now.
This seems to be an expression as the hints link "Níl ann ach" together to mean "It is only"
You could always try to report it to see if they think that it is possible.
But (no pun intended) "we are but boys" was accepted in a similar question. Though not a native English speaker I am well aware of the fact that "only" is a more common expression for what is being said in Irish, but where is the difference? "But" in the English phrase sounds poetic/archaic to me in both cases ("mouse" and "boys").