The auto attendant does not offer local PSTN dial tone. If you select Option 2 "make a new call", dial an Irish number followed by #, then it should work. By default PSTN is the Primary Line, so you do not need to dial any special codes, simply dial the number as if you were in Ireland.
You may find it slow to connect. This is because the OBi devices are set with default DigitMaps that suit North American dialling formats. The OBi110 default Line Port DigitMap (used for PSTN) is:
(xxxxxxxS4|1xxxxxxxxxx|xx.)
This matches seven digit number (adds a four second delay) and eleven digit numbers starting with "1". Everything else is dealt with by xx. which means any number of any digits. The problem is when using xx. OBi waits for ten seconds after the last digit to make sure that you have finished inputting digits, hence the delay in connecting.
There are lots of posts here about changing DigitMaps to suit local number formats should you feel like digging deeper!