The Irish mobile number will be something like 08X XXX XXXX. You need to dial it in full international format, though, which means:
00 = international access prefix from UK
353 = country code for Ireland
8X = Irish mobile prefix, dropping the leading '0' trunk code
XXX XXXX = the rest of the mobile number
Dial 00 353 and then the landline with its area code and leaving out the first zero.
Replace 0 with +44, or with 00 44 from a landline phone.
Dial the UK mobile number exactly the same way you do when the mobile is in the UK.
You would dial 011 353 and then the mobile number or landline number in Ireland, without its first zero.
You can not have two devices having the same number, what you can do instead is transfer incoming calls to the landline to your mobile. In other words, when someone is calling your landline it will instead be redirected to your mobile device.
00 1 from a landline or +1 from a GSM mobile phone, followed by the area code and number.
All Irish (Republic) mobile phone numbers start with 08 and the following digit denotes the network operator the number was originally registered with. Example 08 5(meteor network) 2993856
Just as you would call an English landline.
The country code 00353 is the international dialing code for Ireland. When calling Ireland from abroad, you would dial +353, followed by the local number. This code is used for both landline and mobile phone calls.
Replace the trunk prefix 0 of the Irish domestic telephone number (mobile or landline) with telephone country code +353 (calling on a mobile) or 00 353 (calling on a landline). These instructions apply for all parts of the UK, even including local calls from Northern Ireland to places just over the border in the Republic of Ireland. Calls from anywhere in the UK to Northern Ireland just dial the UK domestic number. The area code for landlines in Northern Ireland is 028, with an 8-digit local number; mobiles from Northern Ireland use the same mobile prefixes as mobiles from the rest of the UK.
To dial any number in the US (landline or mobile) from the UK, dial 00 1 followed by the area code and number.
No - calls to mobile phones form a landline are charged at premium rates. The actual cost-per-minute will have been set by your landline provider.