Just a quick question, is there more than one HopOn/HopOff for Dublin and if so, which is your favorite?

I think there is just the one.

There are at least two. One is a green bus and one is a red bus. About same route and same cost. Very helpful to get to some of the sights too far from downtown to walk easily.

Dublin City Hop-On/Hop-Off Tour - The city's main transport company, Dublin Bus, operates this open-top double-decker bus providing continuous narrated tours (1 hr. 15 min.) and hop-on/hop-off stops at 19 different locations. Tickets are valid for 24 hours. Daily, year-round, from 9:30am. to 5pm. or later. Buy tickets on board, at Dublin Tourism Centre or from Dublin Bus, 59 Upr. O'Connell St., Dublin 1; tel. 01-873-4222; web site www.DublinBus.ie

Dublin Tour Hop-on/Hop-off Tour - Open-top red double-decker bus offers a 90-minute tour with live commentary and 20 hop-on/hop-off stops including Phoenix Park. Tickets are valid for 24 hours. Daily, year-round (except Dec. 16-27 and March 17), from 9:30am to 5pm or later. Buy tickets on board, at hotel concierge desks, House of Ireland, or from the Dublin Tourism Office. Operated by Irish City Tours, 33 Bachelor's Walk, Dublin 1; tel. 01-872-9010; web site: www.irishcitytours.com.

We did the DublinBus tour on a lark one morning. I gather that the driver you get can have some impact on how you feel about it - ours was entertaining, despite the cornyjokes and silly stories. While I don't really encourage anyone to take the tour, I'd also not discourage anyone that was slightly interested. We did find that it gave us a decent overall feel for how the city was laid out, and distances between things that we were interested in getting back to later.

Be warned that it can take some time to do the entire circuit, as some stops can have a lot of people boarding and paying.

when we went...only one went to kilmainham jail so that is what we chose...if that is important to you, check before boarding

There are a few different ones. CityTour by Dublin Bus (green bus) is best - the drivers do the commentary and are very entertaining and helpful. You can also book tickets in advance on www.dublinbus.ie . I think there is a discount if you book on line.
They are also more reliable for turning up especially as the day nears an end.

The hop on hop off bus system is a terrific way to get around the City and go from one site to the next. It really is very handy. I've used the red bus the my last two trips and enjoyed the live commentator. Yes the jokes are corny..but I still laugh. I especially laugh after visiting the Guiness Brewery. Have a great trip.