Ideally I would like a supplier that has some sort of commitment to looking after existing customers rather than giving inducements to new customers and then fleecing them after the time period is up.
I don't think that any of them offer such a commitment.
There's a huge reliance on customer inertia to make money in these markets as far as I can see.
Does Bonkers cover all suppliers?
I am pretty sure that it does and it is generally kept up to date.
I'm a big fan of it for shopping for gas/electricity services.
I'm not so sure about their financial sections though - I don't think that they're as comprehensive, up to date, reliable.
What about One Big Switch? ( I did sign up for them but had to ask them to stop bombarding me with emails)
Let bonkers.ie check and compare them for you.
uswitch.ie is another comparison site but they seem to insist on some form of registration now and it won't even accept my phone number now. This wasn't required in the past so that's disappointing.
You can even switch/sign up for most offers through these sites without having to go off to the provider's website or calling them.
Is it cheaper to go directly to the suppliers?
You can always try getting additional discounts directly from suppliers - especially if you are an existing customer considering a switch to another supplier - but your mileage may vary. I did this with Bord Gais recently but after emailing and calling them and being promised a callback they never bothered to call me back and so I switched to a cheaper provider anyway.
Is it better to get both gas and electricity from one supplier or from separate suppliers
Not according to bonkers.ie.
Be wary of claims of "10% discount" or whatever - that's simply a discount from a provider's own standard UNIT rates, says nothing about their standing charges (which vary) and means nothing in terms of how they compare across the market.
I'd definitely recommend using bonkers to compare the market.
In fact I don't think that you need to do anything else other than decide if you want the (small?) hassle of taking gas and electricity from separate providers for higher savings and if you want to try negotiating even lower charges directly.
Make sure to review stuff like this (and any other regular bill) at least annually if not even more frequently. Some packages will not have an early termination penalty, some do (usually c. €50) and even those that do can be beaten even when the penalty is factored in.
When using the likes of bonkers.ie look at your old bills, work out your average annual usage in terms of gas/electricity units and then punch the details in and hit compare. E.g. get your bill for, say, July 2014 and check the meter reading at that date. Then get the bill for, say, July 2015 and check the meter reading. Subtract the former from the latter to get your annual usage. Obviously you can average over more years or a longer period of time if you want. Those with night and day electricity meters need to do this for both meters and also calculate what % of usage is night versus day. DON'T take your last bill and multiply by 6 or 12. That will skew the annual results (e.g. lower usage in Summer, higher in Winter).
In short - particularly with bonkers.ie, comparing the market and switching is easy and everybody should do it on a regular basis.
Hope this helps.
P.S. I have no affiliation with bonkers.ie other than as a satisfied customer.