Estimates put the total quantity of waste involved at more than 30,000 tonnes.
That amount would fill twelve Olympic-sized swimming pools.
The latest site at Galbally, near Dungannon, is the third such find in the area since the Autumn.
Experts from the Northern Ireland Environment Agency (NIEA) are now linking it to two other dumps discovered previously at Ballygawley and Sandholes.
Derek Williamson, from NIEA's Environmental Crime Unit, described the latest find as "significant".
He said: "Given the nature of the waste deposited, NIEA is currently linking five ongoing investigations in the east Tyrone area.
"We also believe the waste originated in the Republic of Ireland."
Mr Williamson said his officials were working closely with the GardaÃ­ to identify those responsible.
The volume of waste at the most recent find is still being assessed.
A man was arrested as part of the operation and was later released pending further inquiries.
NIEA has appealed for anyone with information on illegal dumping to contact them.
Investigators say parts of County Tyrone are being used by "unscrupulous criminals" to bury waste following the discovery of a fifth illegal dump there.