The 22-year-old joined the Toffees in a £13 million switch from Wigan and, along with fellow late recruit Gareth Barry, must fill the boots of the Manchester United-bound Marouane Fellaini, whose £27.5 million departure paved the way for his transfer.
"I was sitting at home and obviously there were phone calls," McCarthy told the club's official website of deadline day. "Once I knew that Everton were interested there was no going back – I wanted to come here, establish myself and get some games under my belt. To be honest, I was speechless and I'm just delighted to be here.
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The Republic of Ireland international will once again link up with Roberto Martinez, who brought him to Wigan as an 18-year-old from Hamilton Academical in 2009, and says that the Spaniard was instrumental in his decision to move to Goodison.
"I worked under him for four years at Wigan," the midfielder continued. "They were four brilliant years and I loved every minute of it.
"All his coaching staff are here and there are a good bunch of boys [in the squad] because I know a few of them. It was a no-brainer, to be honest. I'm delighted to get it all over.
"He's told me to come here and work hard. I'm not guaranteed my place - rightly so - but I will come here and give it my best shot. I'm over the moon. I'm just delighted to be here now."