Mullagh, County Meath, is in area code 01 with Dublin. (internationally, +353 1)
Mullagh, County Cavan, is in area code 046 (+353 46), along with some parts of County Meath.
Although there were special "shortcut" codes for dialling calls to Ireland from the UK some years ago, they have been discontinued. All calls from the UK (including Northern Ireland) to the Republic of Ireland must now use the full international number.
(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.)
The country code and area code of Mullingar, Ireland is 353, (0)44.
Ireland does not have zip codes. For something going to Mullingar, you would end the address as follows: Mullingar Co. Westmeath Ireland For an internet form looking of a zipcode for anywhere in Ireland, just use IE as the code.
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.
The country code and area code of Ennis, Ireland is 353, (0)65.
The country code and area code of Clonmel, Ireland is 353, (0)52.
The country code and area code of Portlaoise, Ireland is 353, (0)57.
The country code and area code of Longford, Ireland is 353, (0)43.
The country code and area code of Dundalk, Ireland is 353, (0)42.
The country code and area code of Castlebar, Ireland is 353, (0)94.
The country code and area code of Drogheda, Ireland is 353, (0)41.
The country code and area code of Naas, Ireland is 353, (0)45.
The country code and area code of Athlone, Ireland is 353, (0)90.