44 is the international country code for the UK (including England). To text a UK mobile phone from the Republic of Ireland dial '0044' or '+44'.. then the mobile number. Omit any zero from the start of the mobile number.
The Republic of Ireland (the south) is country code +353, and mobile numbers begin with +353 8.
Northern Ireland is part of the United Kingdom, in country code +44, with mobile numbers beginning with +44 74 through +44 79.
In either case, you drop the trunk prefix 0 from the beginning of the domestic-format number. For example, Irish mobile number 08x 1234567 becomes +353 8x 1234567, and UK mobile number 07700 900123 becomes +44 7700 900123.
To send a text to either one from a GSM cellphone, enter the number in international format, beginning with the plus sign. If you have a non-GSM cellphone, replace the plus sign with the international access prefix 011 when calling from North America (USA, Canada, etc.).
00353 then the Irish number minus its leading zero.
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.
Dial 00 353 and then the landline with its area code and leaving out the first zero.
The country code and area code of Carlow, Ireland is 353, (0)59.
The country code and area code of Letterkenny, Ireland is 353, (0)74.
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 Dundalk, Ireland is 353, (0)42.
The country code and area code of Ennis, Ireland is 353, (0)65.
The country code and area code of Portlaoise, Ireland is 353, (0)57.
The country code and area code of Clonmel, Ireland is 353, (0)52.
The country code and area code of Longford, Ireland is 353, (0)43.