He spent his entire professional career with Irish provincial team Connacht and represented Ireland at test level.

In 2000, he was part of the Galway team that won the All-Ireland Minor Hurling Championship, an unused substitute in the final against Cork.

Other members of the squad included Damien and Kevin Hayes, Ger Farragher, Shane Kavanagh, Fergal Moore, Richie Murray and Tony Óg Regan, all of whom went on to play in All-Ireland Senior Hurling Finals with Galway.

Hurling was Muldoon's primary sport growing up, and he idolised Galway's five-time All Star centre forward Joe Cooney.

[6] Having won an All-Ireland Under-18 Cup with Nenagh Ormond Muldoon moved to Galway city to study, taking an Industrial Engineering course in the Galway-Mayo Institute of Technology.

[10][12] Bradley named Muldoon as Connacht captain ahead of the 2008–09 season, replacing Andrew Farley in the role.

In Muldoon's first season as captain, the team made it past the group stages, but were beaten in the quarter-finals by Northampton Saints.

[12] In the 2009–10 Celtic League, Muldoon played 14 games for Connacht, starting in each of them as the province finished bottom of the table for a third successive year.

In the quarters, they faced the French Top 14 side Bourjoin, beating them 23–20 with a late Miah Nikora drop goal.

Connacht advanced to the semi-finals where, on 30 April 2010, they faced a Toulon team featuring the English fly-half Jonny Wilkinson.

He started all six of the team's games including the bonus point defeats to Gloucester and the victory over Harlequins in the return leg in Galway, which was Connacht's first ever Heineken Cup win.

[10] Summer 2013 saw Elwood leave his post as head coach, replaced by former Samoa international Pat Lam.

[18] Muldoon started out the year sharing the captaincy with both Duffy and Connacht's longest serving player, Michael Swift.

[19] After a poor run of form culminating in a 43–10 defeat against Edinburgh, Craig Clarke, former Super Rugby winning-captain with the Chiefs, was made Connacht's team captain with Muldoon and the others continuing to "lead off the field".

[20] Muldoon reached the milestone of playing over 200 times for the province in the 2013–14 season, with the landmark appearance coming on 13 September 2013 against the Cardiff Blues in the Pro12.

[22] Following the forced retirement of Craig Clarke due to persistent concussions at the end of the 2013–14 season,[23] Muldoon was once again handed the captaincy in August 2014.

[29] Pat Lam left Connacht for Bristol in summer 2017, and was replaced by Kieran Keane,[30] who kept Muldoon on in his role as captain.

The result was a 47–10 win for Connacht, with Muldoon scoring his last ever points for the province by converting the team's final try of the day.

[6] With the under-21s, Muldoon played alongside Rory Best, who later captained the Irish senior team and Tommy Bowe, who went on to become the second-highest international try-scorer for Ireland behind Brian O'Driscoll.

He made his debut for the second-tier side on 3 June 2006 when he started against the United States in a 2006 Churchill Cup pool game.

[42] He featured in the pool stage against Canada and the final against England Saxons, scoring a try in the latter, as Ireland won the tournament.

[38][43] On 31 January 2010, he started for the A side, now renamed the Ireland Wolfhounds, against England Saxons in a warm-up game for the 2010 Six Nations, which ended in a 17–13 defeat.

[38][49] In 2017, Muldoon was called up by Robbie Deans for the international invitational team the Barbarians, ahead of their game against Tonga in Thomond Park during the November series.

[51] It was announced in April 2018 that Muldoon would be taking over the role of defence coach, at newly-promoted Premiership outfit Bristol from the 2018–19 season.