Since lough is an noun not a verb, there is no past tense of
lough. The past, present and future of lough is lough. For example
I had been swimming in an lough before or I want to go swimming in
a lough.
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There are three major lakes on the River Shannon - Lough Allen,
Lough Ree and Lough Derg. Lough Derg (13,000 hectares/32,000 acres)
is the largest of the three lakes and is completely navigable.