Storage/Underfloor heating for dummies.

OP your description is a bit vague

If you post up some pictures of your fuseboard and also the thermostats you may get some more help.
Whereabouts in the country are you located?

From your description I reckon you have
1) Storage heating controlled by the controller on the fuseboard (710-2)
2)A boost circuit controlled by the 131 thermostat.

The idea is that the floor stores heat at night during the ESB offpeak and then radiates this heat throughout the day. This is controlled and regulated by the 710-2.

The 131 thermostat is then used if you need to boost/supplement this heat as required.
Note: The boost/131 circuit does NOT use offpeak electricity.

details on the 710-2 can be found here
ftp://cits.devi.com/Internet/08095672-A-R-IE.pdf

and also here
http://www.boards.ie/vbulletin/showthread.php?t=2056766579
http://www.boards.ie/vbulletin/showthread.php?t=2056092781

Your system is as described above with a 750 controller rather than the 710.
User manual
http://www.gaiacs.com/content/assets/Installation_&_User_Instructions_for_Devireg_700_750_to_754.pdf

Ignore all details re day settings, in the manual, as this does not apply in Ireland.

Devi no longer exist in Ireland and Gaia (www.gaiacs.com) in the UK are formerly Devi Electroheat.
If your still unsure it may be worth giving them a call and they should be able to walk you through the use and operation.