Which estate\area in Lucan is your house?
In general the prices in Lucan have gone up 80-100k in the last 18 months, I remember looking at 3 bed semis for 280 back in Oct 2004, they are now 360+.
If I was in your position I would keep the house in Lucan and rent it. If you can find out the exact location of the proposed Luas line in Lucan (due maybe 2012!) then if the house is near it then keep it as it will be a good investment.
Lucan has always been a bit lower priced than other areas, depending on which estate\area of Lucan you are in I would expect to see prices continue to rise as they catch up with other similar areas in Dublin.
If the proposed Technology Campus or whatever it is in Clondalkin goes ahead Lucan will benefit as there will be 2000+ jobs created there.
Depends where you want to buy your new place and where your existing place is in Lucan and if the rent will cover the replayments on your existing place.