How do you call a UK mobile number from a Ireland landline?

For many years, there were special "shortcut" codes to dial some parts of the Republic of Ireland from the UK. However, you now must dial the number in full international format.

Replace the trunk prefix 0 of the Republic of Ireland number with telephone country code +353. If you are calling on a mobile phone, just enter the number in international format, beginning with +353 (including the plus symbol). If you are on a landline phone, replace the trunk prefix 0 with 00 353.

Note that these instructions apply even if you are making a local call from Northern Ireland to a nearby part of the Republic of Ireland, such as from Newry to Dundalk.