Clark captained England at under-18, under-19 and under-20 level, but in October 2010 declared his ambition to play for the Republic of Ireland, the country of his parents' birth.

[5] He was called up to the national team a month later for the friendly match against Norway, and made his full debut against Wales on 8 February 2011.

He was given a squad number for the 2008–09 season and named on the bench for Villa's 2008–09 UEFA Cup match away at CSKA Moscow, but remained an unused substitute.

With Curtis Davies unavailable through injury, the 19-year-old was named in Villa's starting line-up, partnering Carlos Cuéllar in defence.

[10] Although the formidable partnership of new signings James Collins and Richard Dunne limited him to the bench, he was tipped to break into the first team on a regular basis within several years.

Clark was also handed a start in Gérard Houllier's first match as new Villa manager, against Blackburn Rovers in the League Cup.

He then started the match against local rivals Birmingham City in midfield and continued in the role against Fulham and Blackpool due to the club's injury crisis.

[13][non-primary source needed] Clark scored his first FA Cup goal for the club against Bristol Rovers on 29 January 2012, with a solo run, including stepovers and a left-footed strike curled into the bottom corner.

After a dip in his form midway through Aston Villa's season, he was dropped to the bench as Lambert preferred a partnership between Ron Vlaar and fellow academy graduate, Nathan Baker.

[20] In November 2019 he said he had previously considered leaving Newcastle United in order to maintain his place in the Irish international set-up.

[28] Clark captained the England U19 team through qualification for the 2008 UEFA European Under-19 Championship, playing in five out of the six qualifiers and scoring a goal.

[35] In Republic of Ireland's opening match at Euro 2016 against Sweden, Clark deflected a ball from Zlatan Ibrahimović into his own net to level the scores.

Wes Hoolahan had earlier given Ireland the lead but Clark's mishap saw them share the spoils as the match ended 1–1.