AN Irish soldier went to fight in Syria with Islamic State’s “worst nightmare” after telling his bosses he was going to travel the world, it is alleged.

The Dublin-based private is being investigated by the Defence Forces for joining the Kurdish YPG militia.

4 Volunteers from Ireland, Germany and the US

It is understood the man’s six-month break fighting against IS with the ­guerrilla combat force was rumbled following a search through his friend’s Facebook.

The long-serving soldier was in Rojava in northern Syria over ­Christmas 2015, having taken a sabbatical from the Army.

A small ­number of Irish people are known to have joined the Kurdish militia — including Corkman Michael Martin, 33, and British Army man Joshua Molloy, 25, from Laois, who was arrested in Iraq last April for fighting with the 50,000-strong group against IS.

But this man is the first Defence Forces member known to have gone fighting with a foreign militia.

Irish News DUBLIN'S FEAR CITY Capital's Level 3 lockdown kicks in with raft of tighter restrictions WATER TRAGEDY Man aged in 50s drowns after getting into difficulty while swimming in Cork THEFT CASE Man who allegedly pulled amputee from wheelchair & took his wallet refused bail EUR IN LUCK One winner of €500k EuroMillions Plus draw as ticket sold in Kildare DOME WORK DELAY Restoring Four Courts dome costs €4m so far & will take 7yrs to complete Covid fight Leo hopeful as Ireland & Dublin infection rate better than other countries HORROR CRASH Toddler fighting for life after incident involving scrambler and quad bike KICK IN THE BALLS League of Ireland, GAA and rugby at financial risk, Dail watchdog warned covid concern Full list of new measures imposed on Dublin after being placed on Level 3 SHOCK CLIP Leo says 'gardai want to speak to me' after woman throws smoothie at him

A high-ranking source told the Irish Sun: “He fought in several operations while he was in Syria, obviously as a trained solider he was valuable in the field.

“He was in Syria for about six months after crossing in from Iraq during the winter when offences against IS were ongoing.

“He served in a unit with Brits and Americans. All foreigners that join the YPG as fighters see ­frontline action — unlike Iraqi ­Peshmerga.

4 The fighting is intense in Syria

“He would have contacted the now-defunct Facebook page Lions of Rojava to organise his trip over.

“He would have been fighting with a pretty primitive AK-47, given by YPG command and operating in a unit run by Kurdish commander Soran.”

The source added: “This guy joined IS’s number one nightmare. The YPG are the most successful force against IS on the ground.”

It’s understood the soldier was involved in the successful Tishrin Dam offensive, an operation to clear all areas in southern Kobani Canton that had been occupied by IS.

Some 319 Islamic State operatives were killed — 219 on the ground.

It’s believed the unit was disbanded after Tishrin operation, and as soldiers have to do a minimum of six months with YPG so he spent much of the later months waiting his time out.

Sources say a probe into the man’s activities was launched after two soldiers were overheard speaking about it in a Dublin barracks.

4 Everyone gets involved in the fighting

One told us: “Initially military police drew a blank. The ­soldier was careful not to post any pics on his private Facebook page but eventually they ­discovered photographic ­evidence on pages belonging to friends of his.”

It’s understood the Irish soldier was quizzed on a number of occasions over two days earlier this month and is unlikely to face any disciplinary action.

At present one fighter from Ireland (not in the Defence Forces) and up to 20 Brits are known to be fighting alongside Kurdish fighters of the YPG (People’s Protection Units) who are engaged in offences against Islamic State near the city of Raqqa which has been held by IS since early 2014.

The YPG emerged as a serious force following the Syrian revolt in 2011, beginning by defending ­Kurdish majority areas — and by then going to war with al-Qaeda affiliates.

By 2014 the group joined the Free Syrian Army to fight IS in Ar-Raqqah province, receiving air support from the United States in Kobani.

In December 2014, YPG went to war with IS at Kobani, using female resistance fighters, which dragged on for four months.

Increasingly the group turned to foreign volunteers, with soldiers from the US and Australian armies signing up, joined by 400 recruits from Europe and South America by the summer of 2015.

Volunteers from the UK, US, Canada, Portugal and Slovenia lost their lives through 2015 and 2016 in Syria in clashes with IS or in Turkish air strikes.

4 Towns and cities are being destroyed

Last June it emerged that Cork native Mr Martin travelled to join the Kurdish Peshmerga on the frontline in Iraq, who operate mainly on a security basis in the region, unlike YPG who are a feared fighting unit.

He said: “I have four children, we’re lucky to live in a safe, stable country.

"I had skills that I knew would be useful to the Peshmerga forces so I volunteered to help them.

"If we in Ireland were faced with a threat like Islamic State, we would want other people to come and help us and our children.”

Michael met up with Mr Molloy who had been fighting in Syria with the Kurdish People’s Protection ­Forces. Mr Molloy was arrested after he travelled to Iraq from Syria.

Mr Martin added: “We saw a comment saying we’re mercenaries hired by foreign armies, which is totally untrue. We didn’t get paid.

“We didn’t go there to fight or kill people. We went to help people through the humanitarian side of things.”

Last year the Irish Sun reported from the frontline with the Kurdish women fighting IS, who said the brutes were terrified of fighting against them. The Defence Forces were unable to comment when contacted by the Irish Sun.