From a post on MacRumors:
O2 is pleased to advise you that we are in the process of implementing new EDGE technology across the UK. This will greatly improve the data speed to 4 to 5 times faster than GPRS on 2G devices such as BlackBerry.
To deliver EDGE, O2 has planned maintenance activities to be carried out on parts of the GPRS network across the UK until the middle of November 2007. This will result in the loss of service to cells that are being upgraded for up to 2 hours at night during the maintenance window from 10pm to 6am.
The full list of EDGE Network upgrades is at that link. Relevant to NI:
Region Area Postcode Date Scotland NI and Borders Belfast BT12 6HR 29/9/07 Scotland NI and Borders Belfast BT14 6NP 29/9/07 Scotland NI and Borders Belfast BT7 2GB 29/9/07 Scotland NI and Borders Belfast BT9 5DY 29/9/07 Scotland NI and Borders Belfast BT4 1DH 29/9/07 Scotland NI and Borders Belfast BT14 7EE 29/9/07 Scotland NI and Borders Belfast BT9 6SY 29/9/07 Scotland NI and Borders Belfast BT2 7BB 29/9/07 Scotland NI and Borders Belfast BT12 7GL 29/9/07 Scotland NI and Borders Belfast BT11 8BJ 29/9/07 Scotland NI and Borders Malone BT9 5JH 29/9/07 Scotland NI and Borders Belfast BT3 9JS 29/9/07 Scotland NI and Borders Portadown Craigavon BT63 5PE 30/9/07 Scotland NI and Borders Antrim BT41 2LP 30/9/07 Scotland NI and Borders Belfast BT36 6UZ 30/9/07 Scotland NI and Borders Portadown BT63 5BA 30/9/07 Scotland NI and Borders Tommesbridge BT41 3RA 30/9/07 Scotland NI and Borders Belfast BT17 0AT 30/9/07 Scotland NI and Borders BELFAST BT17 0HD 30/9/07 Scotland NI and Borders Carrickfergus BT38 9DE 30/9/07 Scotland NI and Borders Upper Ballinderry BT28 2PQ 30/9/07 Scotland NI and Borders Antrim BT41 4PR 30/9/07 Scotland NI and Borders Carn Money BT36 6QD 30/9/07 Scotland NI and Borders Milltown BT28 3SL 30/9/07 Scotland NI and Borders Broughshane BT43 7HJ 30/9/07 Scotland NI and Borders Ballymena BT42 3HJ 30/9/07 Scotland NI and Borders Belfast Int Airport BT29 4DW 30/9/07 Scotland NI and Borders Newtonabbey BT36 7LQ 30/9/07 Scotland NI and Borders Belfast BT15 4AW 30/9/07 Scotland NI and Borders LISBURN BT28 2UR 30/9/07 Scotland NI and Borders Antrim BT41 1PG 30/9/07 Scotland NI and Borders Craigavon BT65 5AG 30/9/07 Scotland NI and Borders Craigavon BT63 5QQ 30/9/07 Scotland NI and Borders Craigavon BT63 5QE 30/9/07 Scotland NI and Borders Larne BT40 2ST 30/9/07 Scotland NI and Borders Lurgan BT67 9JD 30/9/07 Scotland NI and Borders Dromore BT25 1PR 30/9/07 Scotland NI and Borders Belfast BT8 7XP 30/9/07 Scotland NI and Borders Ballyclare BT39 9BB 30/9/07 Scotland NI and Borders Carrickfergus BT38 9DN 30/9/07 Scotland NI and Borders Sherriffs Mountain BT48 0JX 3/10/07 Scotland NI and Borders Draperstown BT45 7ES 3/10/07 Scotland NI and Borders Dungannon BT71 6SJ 3/10/07 Scotland NI and Borders Londonderry BT48 7AY 3/10/07 Scotland NI and Borders Castledawson BT45 8DU 3/10/07 Scotland NI and Borders Moneymore BT45 7NZ 3/10/07 Scotland NI and Borders Derry BT48 8PY 3/10/07 Scotland NI and Borders Londonderry BT47 6JZ 3/10/07 Scotland NI and Borders Londonderry BT47 5FX 3/10/07 Scotland NI and Borders Londonderry BT47 3LZ 3/10/07 Scotland NI and Borders Cookstown BT80 8HX 3/10/07 Scotland NI and Borders Armagh BT60 1JD 27/10/07 Scotland NI and Borders Omagh BT78 1PL 27/10/07 Scotland NI and Borders Woodview BT61 9HL 27/10/07 Scotland NI and Borders Enniskillen BT74 7HR 27/10/07 Scotland NI and Borders Omagh BT79 7HT 27/10/07 Scotland NI and Borders Omagh BT78 5LU 27/10/07 Scotland NI and Borders Tandragee BT62 2EF 28/10/07 Scotland NI and Borders Newcastle BT33 0LN 28/10/07 Scotland NI and Borders Kilkeel BT34 4BH 28/10/07