The Irish code is 00353 and then you drop the first number of the area code/mobile code, which will be a 0
No area starts with 857 in Ireland. It could either be an 085 number, which is a mobile phone or 057. See the related question below.
This is the start of a mobile (cell) phone number.
The country code and area code of Letterkenny, Ireland is 353, (0)74.
The country code and area code of Carlow, Ireland is 353, (0)59.
Dial 00 353 and then the landline with its area code and leaving out the first zero.
Just enter the UK mobile number exactly the same way you would when that mobile is in the UK.
The country code and area code of Athlone, Ireland is 353, (0)90.
The country code and area code of Castlebar, Ireland is 353, (0)94.
The country code and area code of Clonmel, Ireland is 353, (0)52.
The country code and area code of Drogheda, Ireland is 353, (0)41.
The country code and area code of Dundalk, Ireland is 353, (0)42.