Bitcoin transactions are irreversible.
This is false.
True statements would be:
Well confirmed bitcoin transactions are prohibitively expensive to reverse. Unconfirmed transactions in some situations can be trivially easy to eliminate.
So when I broadcast a txn to the network, with payment to a merchant, wouldn't the merchant receive the bitcoin that I have sent him even if there is no confirmation..!
No. He might receive the transaction, but he wouldn't know for certain if he will have the ability to spend those bitcoins in the future.
How can I double spend the bitcoin (this time with a higher txn fee),
You simply create a new transaction that spends at least one fo the same inputs and convince a miner (or mining pool) to confirm it for you.
since my public key would get debited and the merchant's public key would get credited as soon as I broadcasted my txn to the network..?
This is completely wrong. Bitcoin NEVER debits or credits ANY public keys. It doesn't matter if the transaction is confirmed or unconfirmed, that's just not how bitcoin works.
Now, you might be using an account at a service (such as Coinbase.com), and that service might choose to debit or credit the account that they provide you, but they can just reverse the debit and the credit in their accounting system if they want to.
You also might be using a wallet that hides the details of how bitcoin works. Such a wallet might present an interface that looks like an account and that updates wallet balances to look like there is debiting and crediting going on. However, this is all an attempt by your wallet to show you something that feels familiar. It has nothing to do with how bitcoin actually works, and can cause confusion if you try to apply your knowledge of account based credit and debit systems to the underlying transaction input and output based blockchain system.