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.
The NEID suggests Níl ansin ach luch
"that's only a minor detail" - níl ansin ach mionphointe
"that's only a minor gripe" - níl ansin ach mionrud
"that's only a red herring" - níl ansin ach scéal thairis "that's only a partial explanation" - níl ansin ach leathmhíniú
"that's only a fraction of his salary" - níl ansin ach cuid bheag bhídeach dá thuarastal