Most trackers on an index require the full investment amount to be deposited up front as a lump sum rather that monthly payments. There may be some out there that suit your needs but I'm not aware of them. However there may be other trackers, not index-linked, which accept monthly payments.
You can invest in a tracker by walking in to a branch. There are probably requirements that you need to talk to a qualified financial advisor in the branch. Also companies like Eagle Star, Canada Life and possibly even An Post now offer these.
As for which index to invest in. The ISEQ will most likely be more expensive, this will means that the participation you get in the ISEQ will be less than the participation that you get in the EuroStoxx, so you might only get 85% of the growth of the ISEQ over 5 years whereas you could get 100% of the growth of the EuroStoxx. I'm making up these numbers but you should get the gist. Also the shorter the maturity the less participation you will get. If you were to choose the FTSE, unless you're paying sterling, you should check if you're exposed to currency movements, some trackers protect you from this.
The ISEQ has way outperformed the Eurostoxx over the last few years (84% price appreciation over the last 5 years compared to 23%), but you have to wonder if that is going to continue will all the positive news coming out of Germany and the concerns about Irish competitiveness and the housing bubble.