Fabric Paint...

in dublin id say the best place to try is Evans, and then possibly kennedys (they do expensive but more specialized paints). unfortunetly ireland dosnt have many specialist stores in fabric paint rather they normaly form part of a larger selection. Glow paint tubes might work for you alough the colours are quite flourscent(you can buy it in evans/ inspiring ideas) and it's quite bumpy. ask kennedys to send you theyr catalogue or give evans a ring a see what they stock. (they should be in the telephone directory) or try http://goldenpages.ie

here's a list of paints you can use for cotton shirts http://www.dharmatrading.com/html/eng/3243-AA.shtml

if you really want a certain kind of paint you might trying googling for stores in northern ireland or buy them online (in bulk it's cheaper).