I think the fourth stand will take Tallaght up to 10,000.
The design of Dalymount looks like it won't make it easy to expand in the future. From memory, its main stands will have sloping roofs - which isn't ideal for bolting onto above.
A 6,000 capacity for Dalymount feels too low, and a bit short-sighted. Especially if the league finally gets a bit of promotion and/or an AIL starts. It could become too small very quickly.