36 days!!
A lot.
Manchester to Newquay, Cornwall is 324.5 miles and should take approx 5 hours 54 minutes according to AA route planner.
1 hr 55 min
Fishguard is in Wales, not Ireland. You'll need to take a ferry to get from Fishguard to Dublin or Vice Versa. There are daily ferries, they take a few hours.
Google Earth calculates the distance as 167 miles - with an estimated driving time of 3 hours 17 minutes.
If traffic is kind to you, you should be able to make Exeter from W-S-M down the M5 in about an hour. Allow hour and half to reach Newquay down A30, then forking right round about Bodmin. The A30 is mostly dual carriageway.
3.5 hours without wee break !
245 miles taking this route:Take M5 to The SOUTH WEST, from Birmingham, to A30 to BODMIN and OKEHAMPTON at J31.Take A30 to A392 NEWQUAY via A39.Take A392 to Newquay.
7hrs
You can fly in about an hour. By road you can drive from a number of ferry ports, like Fishguard, Holyhead or Liverpool having got a ferry. If you went to Fishguard, you would have had to drive a bit more before leaving Ireland as the ferry is not from Dublin, but Fishguard is nearer to London than the other ports. It could be anything from 3 to 7 hours by ferry and a 3 to 4 hour journey to London from whichever of the ports you arrived at after that.
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