What is Telephone code ringing from north to southern Ireland?

To call from the Republic of Ireland to England, you must use the full international format, including country code +44 for the UK. Replace the leading 0 of the UK number with 00 44.

Note that there is a special shortcut code for calling from the Republic of Ireland to Northern Ireland landlines. Replace the area code 028 with the special code 048. For mobiles from Northern Ireland, use the full international format, as above.

To call from the UK, including Northern Ireland, to the Republic of Ireland, you must use the full international format, including country code +353 for Ireland. Replace the leading 0 of the Irish number with 00 353.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)